d/Base Theories Lewis Theory

d/Base Theories

Lewis Theory

Acids are electron pair acceptors

Acids are proton-donors

ases are electron pair donol

Bases are proton-acceptors *

Arrhenius Theory

Acids ionize in water and produce hydrogen ions

Bases dissociate in water and produce hydroxide ions

Writing and Naming Acids

1. Complete the following table.

Acid

Formula

Binary Acid Ionization Strong

Acid Name Acid

Formula or Constant at or

Acid

Formula Ternary Weak

1. H2SO4

2. Hypof luorous

acid

3. CHSCOOH

4. HCI

5. Chloric acid

6. H C I O 4

7. Hydrofluoric

acid

8. Boric acid

(B0 3

3 -)

9. H I O 3

10. H 2 0

11. Carbonic

acid

12. Sulfurous

acid

13. H3PO4

14. HCIO

15. Chlorous

acid

16. Hydroiodic

acid

17. HBr

18. H 2 S

2. Write the balanced ionization equations for acids 1 to 5.

pH Stoichiometry: Strong Acids & Bases

Calculate the [HteO*], pH, pOH, and [OH-] for a solution of

0.006 mol/L of hydroiodic acid. Consult tables for Ka.

•=>

pH Stoichiometry: Strong Acids & Bases

26 g of sodium hydroxide in 1 50 mL of solution unclogs a drain.

What is the [OH - ] , pH, and pOH of the solution? Should you wear

gloves? Why?

Percent Ionization of Acids

Percent ionization is the extent to which a weak acid

separates into its ions in aqueous solution.

[H30+]eguilIbnum ^ i Q Q %

[HA]initial

Sample % ionization problem

Calculate the percent ionization of 0.1 00 mol/L phosphoric

acid solution at 25°C. For polyprotic acids, always use K a i .

2 3 %


A 0.1 0 mol/L solution of methanoic acid has a pH of 2.38.

Calculate the percent ionization of the acid, (formula HCOOH)

r


Calculate the acid ionization constant, K a , of 0.1 00 mol/L

acetic acid with an ionization of 1.30%. ( C H 3 C O O H )

1.7 x 10-5

mt

Neutralization Reactions

When solutions of acids and bases are combined they react

according to a neutralization reaction.

Write the balanced neutralization equation for the following

reactions:

1. Sulfuric acid + lithium hydroxide

2. Phosphoric acid + calcium hydroxide

<=> 3. Nitric acid + strontium hydroxide

4. Boric acid + aluminum hydroxide

Strong Acid with Strong Base Titrations

Titration is a technique designed to accurately determine

stoichiometric values when a substance with a known concentration

and volume reacts with another substance.

Changes in pH may be monitored throughout the titration and a

graph of pH versus volume of titrant plotted.

Calculate the pH values every 1.00 mL when 1 0.0 mL of 0.1 00 M

HCI is titrated up to 1 2.0 mL with 0.1 00 M sodium hydroxide

solution.

Date:

Assignment:

From: To: Page No.

Form 4A-BW. © 2000 Mathematics Help Central http://www. mathematicshelpcentral. com

Equivalence is the point in a neutralization reaction where the

moles of acid are stoichiometrically equal to the moles of base.

In the reaction of a strong acid with a strong base, pH at equivalence

is 7.0.

Calculate the pH of the solution at 1, 2, and 3 mL after equivalence.

pH Titration Question

Calculate the pH at each of the following points in a titration of

0.05 M lithium hydroxide with 25.00 mL of 0.1 0 M hydrochloric

acid:

a) before any base titrant has been added

b) after 5.00 mL of base added

c) after 50.00 mL of base added

d) after 51.00 mL of base added

1 0.00 mL of 0.1 5 M sodium hydroxide is titrated to an endpoint

with 0.1 0 M sulfuric acid.

a) What is the volume of acid at neutralization?

Calculate the pH after:

b) 4 mL of acid is added

c) 10.00 mL of acid is added

Calculate the concentration of acetic acid in a 25.00 mL sample

of vinegar if 37.50 mL of 0.10 M sodium hydroxide completely

neutralizes the sample.

Appendix 4.5.e Teacher Support Mate r i a l T i t r a t i o n Curves

Plotting the pH of the solution during an acid/base titration generates a "titration curve". The general shape o f the curves generated in a series o f titrations may be grouped into families, according to the solution titrated and the titrating solutions. Typical examples o f the general shapes are illustrated for four classes o f titration:

I : A solution of a strong acid t i t ra ted w i t h a solution of a strong base.

In this example 25.0 mL of 0.100 mol I . ' aqueous solution o f hydrochloric acid, HC1, (a monoprotic

strong acid) is titrated with a 0.100 mol L r 1 aqueous solution o f sodium hydroxide, NaOH(aq) (an ionic

hydroxide). The equation representing the reaction may be written as:

HCl(aq) + NaOH(aq) NaCl(aq) + H 2 0(1 )

or as the net ionic equation: H } O f (aq) + OH (aq) > 2 l h ( ) ( l )

A t equivalence, moles HC1 originally present = moles NaOH added

Volume MCI original x Concentration HC1 solution = Volume NaOH added x Concentration NaOH solution

A t equivalence, [ H ^ O ' ] = [OH"']; [Na + J = [ C r j ; and since Na" (aq) is a weaker acid than water and CI is

a weaker base than water, the solution is described as "neutral". A t 25°C, the pH w i l l be 7.0 (under the usual set o f assumptions). The expected titration curve is shown in figure 1 below.

pH

14

12

10

8

6

4

2

0

1 «

~ equivalence ) _ _ ^ )

} ^equivalence

1 1 1 1 1 1 1 1 1 1 1 1^1 1 1 1 1 1 1

14

12

10

pH

equivalence

->

i i i i i ' i i i i i i

10 20 30

Volume NaOH(aq) /mL

40 10 20 30

Volume HCI(aq) /mL

40

Figure 1: 0.1 M HCl(aq) vs. 0.1 M NaOH(aq) Figure 2: 0.1 M NaOH(aq) vs. 0.1 M HCl(aq)

Appendix 4.5.c Titration Curves (Contd.) Teacher Support Material

I I . A solution o f a strong base w i t h a solution of a strong acid. This is analogous to the strong acid/strong base titration except that the acid is the independent variable (i.e., is added from a burette or equivalent). The example chosen is the titration o f 25.0 ml o f a 0.100 mol/L aqueous solution o f sodium hydroxide with an 0.100 mol/L solution o f hydrochloric acid. The expected titration curve is shown in figure 2 above. For both cases I and I I , the end point (assumed the equivalence point) is found at the steepest part o f the curve, the inflection point where the curve changes direction.

I I I . A solution of a weak acid w i t h a solution of a strong base

In this example 25.0 mL of 0.100 mol I . ' aqueous solution o f acetic acid, HCH3CO2, (a monoprotic weak

acid with A' a = 1.8 x 10^ 5 mol L " 1 ) is titrated with a 0.100 mol L~~' aqueous solution o f sodium hydroxide,

NaOH(aq) (an ionic hydroxide). The equation representing the reaction may be written as:

H C H . C 0 2 ( a q ) + NaOH(aq) -> NaC H 3 C 0 2 ( aq ) + FI 20(1)

or as the net ionic equation: H C H 3 C 0 2 ( a q ) + OH - (aq) -> CH 3 C0 2 ~(aq) + H 2 0(1 )

(For a weak acid, the acid o f highest concentration in an aqueous solution is the undissociated acid, not the hydronium ion.) A t equivalence, moles H C H 3 C 0 2 originally present = moles NaOH added

A t equivalence, [ H C I l 3 C 0 2 j = [OH ]; [ N a + ] = [ C I I 3 C 0 2 ); and since N a + (aq) is a weaker acid than water

while CT L , C 0 2 is a stronger base than water, the solution wi l l be basic. A t 25°C, the pH w i l l be greater

than 7.0 (under the usual set o f assumptions). The expected titration curve is shown in figure 3 below.

14

12 \ -

10 h

pH 8 h

pH equivalence - - - - - - 9

pH = pK

excess NaOH

PH

10 20 30

Volume NaOH(aq) /mL

40 10 20 30

Volume NaOH(aq) /mL

40

Figure 3: 0.1 M HOAc(aq) vs. 0.1 M NaOH(aq) Figure 3(b) The end point (assumed the equivalence point) is found at the steepest part o f the curve, the inflection point where the curve changes direction.

Titration Curves Student Activity

14

12

10

pH

J I L J I L I I I I I I I J I I I

10 20 30

Volume NaOH(aq) /mL

40

A student pipetted 25.0 mL of an aqueous solution o f an unknown acid into a conical flask, added 25 m l . o f

water, and then titrated the resulting mixture with a standard 0.0985 mol L ' solution o f aqueous sodium

hydroxide, measuring the pH of the mixture after each addition. The above graph shows the titration curve

obtained.

Using this graph, answer the fol lowing questions:

1. What was the concentration o f the unknown acid solution?

2. What would be a suitable indicator for this solution?

3. Assuming the unknown acid is a weak monoprotic acid, estimate its ATa value.

Appendix 4.5.f Titration Curves (Contd.) Student Activity

14

pH

2 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

0 5 10 15 20 25 30

Volume NaOH(aq) /mL

A student weighed 0.225 g o f a solid unknown acid into a conical flask, added about 50 m l . o f water, and

then titrated the resulting mixture with a standard 0.1245 mol L ' solution of aqueous sodium hydroxide,

measuring the pH of the mixture after each addition. The above graph shows the titration curve obtained.

Using this graph, answer the fol lowing questions:

1. What would be a suitable indicator for this solution? 2. Assuming the unknown acid is a weak monoprotic acid, estimate its A"a value.

3. Estimate the molar mass o f the unknown acid.

4. Would the results obtained have been different i f she had added 100 m l . o f water'.' Explain your answer.

Appendix 4.5.f Titration Curves (Contd.) Student Activity

10

pH

9

8

5

4

6

7

3

2 l l l l I I I I I l I I t l I I l I I I I I I I I I 1 I l l I l l l l I

0 5 10 15 20 25 30 35

Volume HCI(aq) /mL

A student weighed 0.0165 g o f a solid unknown base into a conical flask, added about 100 m l . o f water,,

and then titrated the resulting mixture with a standard 0.02635 mol L r ' solution o f hydrochloric acid, measuring the pH o f the mixture after each addition. The above graph shows the titration curve obtained. Using this graph, answer the following questions: 1. What would be a suitable indicator for this solution? 2. Assuming the unknown base is a weak monoprotic base, estimate its value. Estimate the A'., o f its

conjugate acid. 3. Estimate the molar mass of the unknown base.

C E x p 2 : V O L U M E T R I C A N A L Y S E - ACETIC A C I D I N VINEGAR

Carefully read the text describing this experiment then complete the pre-laboratory exercise (c£ pages 41 and 42) to

prepare for the laboratory experiment This experiment focuses on techniques and must he done individually. For accurate results

you must measure carefully to the limits of the equipment used and employ only clean glassware. Your mark will depend on

technique and accuracy of results. Be sure to follow till safety regulations.

OBJECTIVES; A . T o prepare a p r i m a r y acid s tandard (potass ium h y d r o g e n phthala te) and use i t to s tandardize a

so lu t i on o f s o d i u m h y d r o x i d e .

B. T o de t e rmine the concent ra t ion o f acetic acid i n a sample of v inegar b y t i t r a t i o n w i t h the

s tandard ized s o d i u m h y d r o x i d e so lu t ion .

INTRODUCTION

Acid-base S to i ch iomet ry

A n y ac id w i l l react w i t h any base. P r o v i d e d ei ther the acid or the base is strong the react ion goes to c o m p l e t i o n . Thus ,

the s t r ong base s o d i u m h y d r o x i d e dissociates comple t e ly to f o r m h y d r o x i d e ions w h i c h react comple te ly w i t h b o t h s t rong

and w e a k acids, accord ing to the equat ions:

OH-(aq) + H+(aq) - > H2CKI) [strong acid]

O H "(aq) + H A ( a q ) <=> H2O (1) + A _ ( aq ) [weak monoprotic acid]

20H - ( aq ) + TfeUaq) o> 2 H 2 O (1) + I > ( a q ) [weak diprotic acid]

F r o m the balanced equations, y o u can see that 1 m o l of h y d r o x i d e i o n reacts w i t h 1 m o l of h y d r o g e n i o n ; 1 m o l o f

h y d r o x i d e i o n reacts w i t h 1 m o l o f weak m o n o p r o t i c ac id; 2 m o l of h y d r o x i d e i o n are r e q u i r e d to neut ra l ize 1 m o l o f w e a k

d i p r o t i c ac id .

Ind ica tors

The p o i n t of exact neu t r a l i za t ion of ac id b y base is de t e rmined b y u s i n g an indicator that is added to the react ion m i x t u r e .

A n acid-base ind ica to r can u sua l ly exist i n t w o f o r m s (acidic, H I n , and basic, I n - ) , and there is an e q u i l i b r i u m be tween

the t w o states w h i c h depends o n the ac id i ty o f the so lu t ion . For example :

H I n o I n - + H +

(acidic) (basic)

For a u se fu l ind ica to r , the acidic and basic f o r m s of the ind ica to r are t w o d i s t inc t ly d i f f e r e n t colors. I n an acidic m e d i u m ,

the ind ica to r exists as H I n a n d has its specif ic color . I n a basic m e d i u m , the ind ica to r exists as I n - and has a d i f f e r e n t

color. Thus , w h e n an ind ica to r color change occurs i n a t i t r a t i on , y o u k n o w that a change f r o m an acidic to basic m e d i u m

(or vice versa) had unde rgone i n the s o l u t i o n i tself mean ing that the analyte had reacted. This is the end-point that y o u

are l o o k i n g fo r . W h i l e the choice of indica tors can be i m p o r t a n t , on ly a trace a m o u n t of i nd ica to r so lu t ion is used and i t

does no t apprec iab ly affect the v o l u m e of base needed to reach the e n d p o i n t i n the t i t r a t i on .

P r i m a r y Standards

V o l u m e t r i c analysis depends u p o n the ab i l i t y of the chemis t to prepare solut ions of an exact ly k n o w n concent ra t ion . A

chemica l substance suitable f o r use as a p r i m a r y s tandard m u s t sat isfy the f o l l o w i n g requi rements :

i ) I t m u s t be r e a d i l y avai lable i n an ex t remely p u r e f o r m (99% p u r e or greater)

i i ) I t mus t be stable u n d e r n o r m a l cond i t ions of storage and use.

i i i ) I t m u s t be reasonably soluble (usua l ly i n wa te r ) .

These requ i rements a l l o w a p r i m a r y s tandard s o l u t i o n to be p repa red b y w e i g h i n g o u t the substance and d i s s o l v i n g i t i n

water . I n a d d i t i o n , i t is desirable to use a substance w i t h a h i g h m o l a r mass (molecular w e i g h t ) to m i n i m i z e w e i g h i n g

002.130 Chemistry Laboratory CExp2 page 33

errors . I n practice, ve ry f e w chemicals meet these requi rements . N a O H C A N N O T be used as a p r i m a r y s tandard

because i t cannot be obtained i n a p u r e f o r m , and i t is n o t stable ( i t q u i c k l y absorbs wa te r f r o m the a i r ) .

One o f the mos t u se fu l p r i m a r y s tandards f o r v o l u m e t r i c analysis of acids and bases is

po tas s ium h y d r o g e n phthalate , KHC8H4O4, o f t e n abbrevia ted as K H P . This c o m p o u n d

is stable and non-hygroscopic . Its m o l a r mass is 204.33 g / m o l and i t is avai lable 99.9%

pure i n the a n h y d r o u s f o r m . Th i s acidic salt is m o n o p r o t i c and reacts s to ich iomet r ica l ly

i n a 1:1 ra t io w i t h the s o d i u m h y d r o x i d e so lu t ion used i n this analysis.

Use of .Glassware

Glassware s h o u l d be clean before u n d e r t a k i n g an analysis (see c leaning procedures i n the Preface). W h e n us ing a buret

(see b e l o w ) , i t is i m p o r t a n t to check that the stopcock is free f l o w i n g b u t does no t leak. Test y o u r bu re t w i t h d i s t i l l ed

water a f t e r i t has been cleaned. Wash t i t r a t i on flasks w i t h w a r m wate r then rinse w i t h d i s t i l l ed wate r be fore repeat ing

trials.

T i t r a t i o n

W h e n it is necessary to f i n d the exact a m o u n t of an acid or base, w i t h at least one of t hem i n so lu t i on , the process o f

titration is o f t e n used.

I n a t i t r a t i o n y o u add a specif ic v o l u m e of one reagent to ei ther a k n o w n v o l u m e or a k n o w n mass of the second reagent.

Then , from the balanced equation, y o u can de te rmine the a m o u n t ( n u m b e r o f moles) of reagent that m u s t be present f o r

comple te react ion, and hence ei ther its concentra t ion or its mola r mass.

A t i t r a t i o n depends u p o n the p r o p e r p repara t ion and use of a buret . A f t e r y o u f i l l the bure t and make sure that i t is

ope ra t ing p r o p e r l y (Figure 1) y o u mus t record y o u r i n i t i a l v o l u m e (Figure 2).

OCOOH

COO " K +

Burets Figure 1: Preparing buret for Titration a. Fi l l ing

Burets are long graduated cylindrical tubes of very uniform bore, with some device to control the f low of l iquid (a stopcock). Burets are calibrated to deliver the measured volume of l iquid starting wi th the tip f u l l o f l iquid. Most common laboratory burets have, a total capacity o f 50 m l , wi th graduations every 0.1 mL.

I f the buret is dirty so that water does not run cleanly down its sides but leaves droplets behind, scrub it wi th detergent and water. Rinse the buret thoroughly two or three times with tap water and then twice with distilled water. Do not forget to run water through the buret tip. Then drain the buret tip thoroughly.

Use a funnel to f i l l the buret wi th about 10 m L of the solution to be dispensed f rom it. Ti l t and rotate the buret tc rinse down the buret walls wi th the solution. Run some of the solution through the buret tip. Then drain the buret and tip thoroughly, discarding the rinsing solution. Repeat this whole procedure a second time. Be sure to have a freely turning stopcock for controlled delivery.

Clamp the buret on its stand, using the special buret clamps. F i l l the buret above the zero mark with the solution and run some rapidly through the buret tip so that no air bubbles remain in the tip. Run the solution f r o m the buret until the solution meniscus is slightly below, the zero mark. After al lowing 20 seconds for the solution level to stabilize, read and immediately record the initial buret level. To facilitate reading, prepare a white card wi th a dark area and place it behind the buret. The reflection o f the dark portion in the meniscus makes the meniscus stand out more clearly, and a reading to 0.01 m L can be estimated. When reading the buret, make sure that the meniscus is at eye level.

b. check for proper delivery

be sure there is no air bubble in tip

N o w place the f lask con ta in ing the measured a m o u n t o f mate r ia l to be t i t r a ted a n d an e n d - p o i n t ind ica to r u n d e r the b u r e t

t i p so tha t the t i p is w i t h i n the v o l u m e of the f lask u n d e r ope ra t ing cond i t ions (Figure 3). A d d a f e w d rops o f an end -

p o i n t i nd ica to r then beg in to t i t rate b y s w i r l i n g the f lask and its contents w h i l e a d d i n g t i t r an t . N o t e the h a n d pos i t ions

s h o w n i n F igure 3. Genera l ly , the color change characteristic of the ind ica to r w i l l disperse r a p i d l y at f i r s t . C o n t i n u e

a d d i n g s o l u t i o n f r o m the b u r e t f a i r l y q u i c k l y u n t i l the color begins to disperse m o r e s l o w l y . A t th is p o i n t make a men ta l

002.130 Chemistry Laboratory CExp2 P a g e 34

note o f the app rox ima te v o l u m e and beg in to a d d s o l u t i o n f r o m the bu re t d r o p b y d r o p . C o n t i n u e this s l o w a d d i t i o n

w h i l e s w i r l i n g the f lask u n t i l one d r o p (or f r a c t i o n of a d r o p ) causes a pe rmanen t co lor change of the ind ica tor . W a i t

about 30 seconds to be sure that the ind ica to r color is pe rmanen t and the bu re t l eve l has s tab i l ized . Record the f i n a l bu re t

r ead ing . (Note : burets genera l ly de l i ve r about 20 d r o p s / m L . Since t i t ra t ions are usua l ly unce r t a in to one d r o p , y o u r

p rec i s ion on a t i t r a t i o n w i l l be of the o rde r o f 0.05 m L . Be sure to record a l l bu re t readings to t w o dec ima l places.)

F igure 3: H o w to con t ro l the f l o w o f so lu t ion f r o m the buret t ip .

The procedure illustrated is for a right-handed person.

Put your left hand on the stopcock part of the buret as illustrated. For most people it is easiest to place your thumb in front and your fingers behind. The tension on the stopcock shoud be such that it can be turned easily but does not leak. A simple flick movement will place the hole in line with the barrel so that solution can flow. It is possible to control this so drops come individually or in a stream. With your thumb and forefinger around the handle you will always be applying an inward pressure to keep the stopcock seated and avoid leaking

At the same time your right hand should control the flask. Swirl it continuously as solution is added from the buret. In this way the solution is always mixed so that there is less chance that you will miss a color change and end-point.

For a left-handed person; the right hand is on the buret and the left hand is used to swirl the flask.

NOTE: There is information about titration procedures on the Chemistry 002.130 U of M web site, specifically in the

Laboratory Section. Check this and additional sources before coming to the lab.

PROCEDURE:

A. Standardization of a Sodium Hydroxide Solution


The solutions of sodium hydroxide provided for this experiment w i l l be approximately 0.1 m o l / L ; you w i l l have to standardize the solution to f ind its exact concentration.

Fil l about % of a plastic 14-teaspoon w i th potassium hydrogen phthalate (KHP) and transfer all of the crystals in your plastic weighing bottle. Take this sample and two labeled 125-mL flasks to the balance room w i t h your data sheet. Record (to the nearest 0.001 g) the combined mass of the sample and weighing bottle. While gently rotating the weighing bottle, t ip i t over one of the flasks in order to transfer about one-half of the crystals into it (this should correspond to a transfer of about 0.5 g of solid). R e w e i g h the remaining sample and weighing bottle. The difference in mass is the sample s ize

for the first titration. Transfer the remaining crystals into the second flask. R e w e i g h the weighing bottle and, by difference, deterrrtine the mass of the second sample.

Obtain about 200 mL of the 0.1 M sodium hydroxide solution in a clean 250 mL Erlenmeyer flask and stopper i t . Rinse your previously cleaned buret w i th a small portion (about 10-15 mL) of the sodium hydroxide solution then discard the rinsing l iquid. N o w fill the buret w i th this solution, eliminating any air bubbles f rom the tip after f i l l ing. Be sure the solution flows evenly f rom the buret. Wait 20 seconds for the l iquid level to stabilize slightly below the zero mark, then read and record the initial volume (V,) of NaOH solution on your data sheet (your reading should be to the nearest 0.01 mL, i.e. to 2 decimal places).

Add about 25 mL of water and 2 or 3 drops of phenolphthalein* indicator to one of your samples of potassium hydrogen phthalate in the 125 mL Erlenmeyer flask. Although all of the solid may not dissolve immediately it w i l l go into solution as the titration proceeds. Add sodium hydroxide solution f rom the buret while swirling the flask and contents. Watch for localized pink coloration. A t the endpoint, one drop of base w i l l cause the color change f rom colorless to pink, so proceed carefully. Once the end point is reached, wait 20 seconds for the level of solution in the buret to stabilize, then read and record the final volume (V f) of NaOH solution.

* Phenolphthalein has been found to be the best indicator for most students; the color change is f r o m colorless ( in acid) to pink ( in

basic solution). The endpoint is signaled by the first pale pink coloration that is reasonably permanent. I f you cannot see this color

change, consult your T .A . for other suggestions o f indicators you might use.

Repeat the titration w i th your second sample. If the results are not essentially the same, titrate a third sample, and i f necessary, more unti l constant results are obtained.

[While your results should not differ by more than 2% to be acceptable, good work w i l l be even better. A quick check on the precision may be obtained by dividing the volume of base titrated by the sample size; the results should be wi th in 2% of each other.]


B. Ac id Analysis i n Vinegar

Obtain between 10 and 20 mL of the vinegar your T.A. has assigned you. Using the 10.00-mL Class A transfer pipet provided, transfer 10.00 mL of the vinegar into a clean 100 mL volumetric flask. Record the code of your vinegar sample on your data sheet. M i x the solution as you f i l l the 100.0-mL volumetric flask exactly to the calibration line w i th distilled water. Invert the closed flask many times to mix thoroughly.

Using a 25.00-mL transfer pipet, transfer an aliquot of this diluted vinegar solution into a labeled 125 mL Erlenmeyer flasks. Add 2 or 3 drops of indicator solution and titrate w i t h your standardized sodium hydroxide solution. The end-point is again determined by a persistent color change upon the addition of one drop of the basic solution. Record all volume readings (V, and V t) to the nearest 0.01 mL. Repeat the titration of this vinegar solution unti l you have three acceptable trials.

MEASUREMENTS, NOTES, and OBSERVATIONS


T A B L E B. D E T E R M I N A T I O N O F A C E T I C A C I D I N V I N E G A R

Standard [ N a O H ] ( f r o m Part A )

T i t r a t i o n : Ind ica to r used

T R I A L

V o l u m e of d i l u t e vinegar t i t ra ted ( m L )

Final v o l u m e reading on buret , V. ( m L )

In i t i a l v o l u m e reading on buret , V, ( m L )

' V o l u m e N a O H so lu t ion used ( m l . )

| C H , C O : H ] in d i l u t e sample ( m o l / L )

[CFLCO-.H] in vinegar sample ( m o l / L )

"'<> Acet ic A c i d bv v o l u m e in v inegar (*)

B r a n d (or code) of v inegar used:

(2)

3 (if needed) 4 (if needed)

(*) Repor t acetic acid concent ra t ion i n v inegar as % acetic acid by volume, as i t is done i n the marketp lace .

T o d o so, assume that densities o f acetic ac id , v inegar so lu t ion , and wa te r are a l l the same at 1.00 g / m L .

A concent ra t ion expressed i n percent b y v o l u m e (% % ) is d e f i n e d as V ( C H X O ? H ) _ n /

—' 3- 2—t x 1 0 0 %

V(Vinegar)

Sample ca lcu la t ion f o r concent ra t ion of acetic ac id i n v inegar ( i n m o l / L and % % ):

(2)

Average % acetic acid by volume in vinegar: ± % (2)

Observat ions :

(1)

DISCUSSION and C O N C L U S I O N (attach another page for further explanations)

002.130 Chemistry Laboratory CExp2

(2)

page 40

Problem Set m : Aqueous Equilibrium

1. Name the following acids, write their ionization equations, and write the Ka expression

for each. (2 points)

a) HCIO

b) HF

c) HCN

d) HBr

2. Order the adds in question 1 from weakest to strongest. (0.5 point)

3. Which acid in question 1 is the strongest electrolyte? The weakest electrolyte?

(0.5 point)

4. Calculate the equilibrium concentrations of all species given a 0.5 M hydrocyanic acid

solution at 25°C. Use tables for IC This is an I C E table question (2 points)

f | j j |

5. Calculate the pH, [H+], [H 3 0+ ] , pOH, [OH"], and percent ionization of a 0.1 M nitrous

acid solution. Use tables for Ka. (3 points)

6. Given a pH of 3.13 for a 0.15 M weak acid solution, HA, calculate the acid ionization

constant, Ka. (2 points)

7. Calculate the pH of a 0.1 AA hypochlorous acid and 0.2 M sodium hypochlorite solution.

Use tables for Ka. (2 points)

(

/ 1 2

T.A. Name:

D a y /Sec t ion :

R o o m / Locker :

Date:____

C E x p 2 : V O L U M E T R I C ANALYSIS - A C E T I C A C I D I N V I N E G A R

D A T A and RESULTS: Record i n f o r m a t i o n o n Tables A and B or i n the space p r o v i d e d .

T A B L E A. PREPARATION O F STANDARD NaOH S O L U T I O N (marks)

T i t r a t i o n : N a O H Sample # Ind ica to r used _ _ _ _ _ _ _ _ _ _ _

Mass o f w e i g h i n g bot t le w i t h K H C g H 4 0 4

Mass o f bo t t l e m i n u s sample: _ _ _ _ _ _ _ _ _

Mass o f K H C 8 H 4 0 4 used: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (1)

T R I A L 1 2 3 (if needed) 4 (if needed)

Final volume reading on buret, V, (mL)

Initial volume reading on buret, V. (ml.)

Vo lume N a O H solution used ( m L )

Concentration N a O H solution ( m o l / L ) |

A v e r a g e [ N a O H ] (± average dev ia t ion ) : ± m o l / L (2)

Sample ca lcu la t ion f o r concent ra t ion o f N a O H so lu t ion :

Observat ions :


T . A . N a m e : _ _ _ _ _

D a y / G r o u p : __

Room, / Locker :

Date:

CExp2: Pre-Lab Exercise for Volumetric Analysis Experiment

To be submitted to your T.A. before you begin the experiment. It will be returned with your report which must be submitted in one week for full credit. Late labs will be penalized.

1. Potassium hydrogen phthalate (KHP) is a weak monoprotic acid.

a) How many moles of NaOH are required to neutralize 0.616 g of KHP? (1)

b) If the neutralization of 0.616 g of KHP requires 31.47 mL of NaOH solution, what is the concentration of the NaOH solution? (1)

2. I t is suggested that phenolphthalein is the best indicator for this titration.

a) What color would the solution be if you added phenolphthalein to a solution that was:

(0.5)

- acidic?

- basic?

b) Why is the first indication of a pink color not necessarily the true end-point?

(0.5)


3. What is the difference between a primary and a secondary standard solution? In this experiment, is the sodium hydroxide solution provided a primary or a secondary standard solution? Explain. (1)

4. If the acetic acid content of wine vinegar is measured as 1.00 m o l / L , what weight of acetic acid would there be in 1 litre of the vinegar solution? Assume that densities of acetic acid, water, and wine vinegar are all the same at 1.00 g / m L . What is the percent by volume (% % ) of acetic acid in wine vinegar (1)

5. A student added NaOH solution unti l the phenolphthalein indicator was an intense pink color (i.e. they overshot the end-point). Explain what effect this would have on the concentration of acetic acid reported for the vinegar sample.

a )


BLA—BLB Theory & Titrations

Bronsted—Lowry Acids and Bases

Arrhenius acids and bases are classified as containing H + ions and OH" ions in water, respectively. According to Arrhenius, the identities of acids and bases are static and do

™t change. H+ions also

exist as H 5 0 2 \ The Arrhenius definition is impractical as it does not account for: ^ Q + Q n c j c y c n

1. the existence of acids and bases in non-aqueous solutions H O +

2. the fact that H + ions exist as rLO +

3. the observed behaviour of some chemicals that act either as acids or bases—called amphoteric or amphiprotic substances—depending on the chemicals with which they are combined.

A broader definition of acids and bases is required to account for the limitations of Arrhenius theory. Note that a

hydrogen ion, A Bronsted—Lowry Acids (BLA) is a substance capable of donating protons, also h + , is simply a called a proton donor. proton!

Bronsted—Lowry Bases (BLB) are described as substances that accept protons, also called a proton accceptor

T h e Amphoter ic Nature of W a t e r

Example of water behaving as a Bronsted base.

HClfej + H20(i) H 3 0+ ^ ; + CI (aq)

H (aq)

Example of water behaving as a Bronsted acid.

N H 3 f e ; + n 2 o 0 ) <-» NHA,„, + OH'

Conjugate A c i d — B a s e Pairs

Conjugates exist for every BLA and BLB pair.

(aq)

Any reactant that acts as a BLA forms a conjugate Bronsted base (CB) on the product side of the equation and any BLB reactant molecule forms a conjugate Bronsted acid (CA) conjugate on the product side of the equation.

BLA BLB CA CB

HN0 3 + H 2 0 <r> H 3 0+ + N0 3

H \AAAW.pembinatrails.ca/shaftesbury/mrdeakin °1_J [email protected]

® (204) 888-5898 _____ Shaftesbury High School, 2240 Grant Ave, Wpg, MB, R3P 0P7

BLA—BLB Theory & Titrations

Sample Problem

Identify all Bronsted—Lowry components (BLA, BLB, CA, CB) in the following equation.

^COlfaq) + HlO(l) <r> (aq) + HCO3 (aq)

Solutions of Weak Bases Weak bases do not react completely with water to form hydroxide ions.

Just like acid dissociation constant values, K„, describe the strength of acids, base dissociation constant values, Kb, are used to quantitatively determine the relative strength of a weak base.

Kb expression and value for pyridine, C 5 H 5 N

Furthermore, Kb values are related to K a and K

K b is determined f r o m K a and K w

Hydrolysis of Anions and Cations Hydrolysis can be defined as a reaction of a catioriTIr anion. Wfh &ater resulting in a pH m or anion wm change of the solution.

Hydrolysis occurs in solutions of dissolved salts containing the anions or cations of weak conjugate acids or bases.

hydrolysis o f CN" and N H 4

4

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d/Base Theories Lewis Theory