AQUEOUS SOLUTIONS

AQUEOUS SOLUTIONSAQUEOUS SOLUTIONS

Solute ConcentrationSolute Concentration

Molecular WeightMolecular Weight = Sum of weight of all atoms in a = Sum of weight of all atoms in amolecule (expressed in Daltons).molecule (expressed in Daltons).

For example: Determine a mole of For example: Determine a mole of CHCH33COOHCOOH

CHCH33COOH COOH 2 C 2 x 12 Da = 24 2 C 2 x 12 Da = 244 H 4 x 1 Da = 44 H 4 x 1 Da = 42 O 2 x 16 Da = 322 O 2 x 16 Da = 32 ------------------------------------------

M.W. Da = 60 g/molM.W. Da = 60 g/mol

AQUEOUS SOLUTIONS

MoleMole = Amount of a substance that has a mass in grams = Amount of a substance that has a mass in gramsnumerically equivalent to its molecular weight in numerically equivalent to its molecular weight in

Daltons.Daltons.For example: To determine a mole of sucrose For example: To determine a mole of sucrose

((CC1212HH2222OO1111).).Calculate molecular weight:Calculate molecular weight:C = 12 DaC = 12 Da 12 Da x 12 = 144 Da 12 Da x 12 = 144 DaH = 1 Da 1 Da x 22 = 22 DaH = 1 Da 1 Da x 22 = 22 DaO = 16 Da 16 Da x 11 = O = 16 Da 16 Da x 11 = 176176 Da Da 342 Da342 DaExpress it in grams (342 g).Express it in grams (342 g).

AQUEOUS SOLUTIONS

MolarityMolarity = Number of moles of solute per liter of = Number of moles of solute per liter of solution.solution.

For example: To obtain 1 M sucrose solution, weigh For example: To obtain 1 M sucrose solution, weigh out out

342 g and add water up to 1L.342 g and add water up to 1L.Advantage of measuring in moles:Advantage of measuring in moles:1. Rescales weighing single molecules in Daltons to 1. Rescales weighing single molecules in Daltons to

grams, which is more practical for laboratory use.grams, which is more practical for laboratory use.2. A mole of one substance has the 2. A mole of one substance has the samesame number of number of

molecules as a mole of any other substance molecules as a mole of any other substance (6.02 x 10(6.02 x 102323; Avogadro’s number).; Avogadro’s number).3. Allows one to combine substances in fixed ratios of 3. Allows one to combine substances in fixed ratios of

molecules molecules

AQUEOUS SOLUTIONS

For example:For example:

How much How much CHCH33COOHCOOH do you needdo you need to make 1 to make 1 liter of a 2 M solution of liter of a 2 M solution of CHCH33COOH?COOH?

Ask yourself these questions:Ask yourself these questions: 1.1. How many grams of this substance are in How many grams of this substance are in

1 mole of 1 mole of CHCH33COOHCOOH? ? 2.2. How many liters are you trying to prepare? How many liters are you trying to prepare?3.3. What is the ultimate concentration you are What is the ultimate concentration you are trying to prepare (M)?trying to prepare (M)?


Answer:Answer:

60 grams/mole x 1 liter x 2 moles/liter = 60 grams/mole x 1 liter x 2 moles/liter =

120 g120 g of of CHCH33COOH COOH is needed to prepare is needed to prepare aa

2 M solution.2 M solution.

AQUEOUS SOLUTIONS

For Example:For Example:

How much How much CaClCaCl22 do you needdo you need to make 750 ml of to make 750 ml of

a 1.5 M a 1.5 M CaClCaCl22 solution?solution?

How many grams/mole? How many grams/mole? 110 g/mol110 g/mol

How many liters? How many liters? .750 L.750 L

What is M? What is M? 1.5 M1.5 M


Answer:Answer:

110 g/mole x .750 L x 1.5 moles/liter = 110 g/mole x .750 L x 1.5 moles/liter =

123 g 123 g

You will need You will need 123g123g of CaCl of CaCl22

AQUEOUS SOLUTIONS


You want to make a 10 M You want to make a 10 M CC66HH1212OO66 solution. solution. If you have 900 grams of If you have 900 grams of CC66HH1212OO66, how , how much can you make? much can you make?

How many grams/mole? How many grams/mole? 180 g/mol180 g/mol

What is the M desired? What is the M desired? 10 M10 M

How many grams total do you have toHow many grams total do you have to

work with? work with? 900 grams900 grams

AQUEOUS SOLUTIONS

Answer:Answer:

You can make 0.5 L of a You can make 0.5 L of a

10 M 10 M CC66HH1212OO66 solution solution


What if the question would have asked What if the question would have asked for for

a 2.5 M Ca 2.5 M C66HH1212OO66 solution? What about solution? What about

a 4 M Ca 4 M C66HH1212OO66 solution? solution?


Answer:Answer:

5 moles / 2.5 moles/liter = 2 L 5 moles / 2.5 moles/liter = 2 L

5 moles / 4 moles/liter = 1.25 L5 moles / 4 moles/liter = 1.25 L


For Example: For Example: What is the What is the molar concentrationmolar concentration of a of a NaOH NaOH solution where you have been given 60 grams solution where you have been given 60 grams of of NaOH NaOH and ask to prepare a final volume of and ask to prepare a final volume of

3 liters?3 liters? What is the amount of grams/mole? = What is the amount of grams/mole? =

40 g/mole40 g/mole How many grams do you have? How many grams do you have? 60 60

gramsgrams What is your desired volume? What is your desired volume? 3 L3 L


Answer:Answer:

60 grams / 40 grams/mole = 1.5 60 grams / 40 grams/mole = 1.5 molemole

1.5 moles / 3 liters = 0.5 moles/liter1.5 moles / 3 liters = 0.5 moles/liter

The molar concentration is 0.5 M.The molar concentration is 0.5 M.

Acids, Bases and pHAcids, Bases and pH

Dissociation of Water Molecules (pH)Dissociation of Water Molecules (pH)

Occasionally, the hydrogen atom that isOccasionally, the hydrogen atom that is

shared in a hydrogen bond between twoshared in a hydrogen bond between two

water molecules, shifts from the oxygenwater molecules, shifts from the oxygen

atom to which it is covalently bonded to theatom to which it is covalently bonded to the

unshared orbitals of the oxygen atom to which unshared orbitals of the oxygen atom to which it isit is

hydrogen bonded.hydrogen bonded.

Figure 3.9 The pH of some aqueous solutionsFigure 3.9 The pH of some aqueous solutions


1. Only a 1. Only a hydrogen ionhydrogen ion (proton with +1 (proton with +1 charge) is actually transferred.charge) is actually transferred.2. Transferred proton binds to an unshared 2. Transferred proton binds to an unshared

orbital of the second water molecule orbital of the second water molecule creating a creating a hydronium ionhydronium ion (H (H33OO++).).

3. Water molecule that lost a proton has a 3. Water molecule that lost a proton has a net negative charge and is called a net negative charge and is called a hydroxide ion hydroxide ion (OH(OH--).).

HH22O + HO + H22O ↔ HO ↔ H33O+ + OHO+ + OH--

Unnumbered Figure (page 47) Chemical reaction: hydrogen bond shiftUnnumbered Figure (page 47) Chemical reaction: hydrogen bond shift


4. By convention, ionization of H4. By convention, ionization of H22O is O is

expressed as the expressed as the dissociationdissociation

into Hinto H++ and OH and OH--

HH22O ↔ HO ↔ H++ + OH + OH--

5. Reaction is reversible.5. Reaction is reversible.

6. At equilibrium, most of the H6. At equilibrium, most of the H22O is O is

not ionized. not ionized.


1. At equilibrium in pure water at 251. At equilibrium in pure water at 25ooC:C:

2. Number of H+ ions + number of OH2. Number of H+ ions + number of OH-- ions. ions.

[H+] = [OH-] = [H+] = [OH-] = 1 1____ M = 10 ____ M = 10 –7–7 M M

10,000,000 10,000,000

*Note that brackets indicate molar concentration**Note that brackets indicate molar concentration*


A solution in which:A solution in which:

[H+] = [OH-] is a neutral solution.[H+] = [OH-] is a neutral solution.

[H+] > [OH-] is an acidic solution.[H+] > [OH-] is an acidic solution.

[H+] < [OH-] is a basic solution. [H+] < [OH-] is a basic solution.


The pH ScaleThe pH ScaleIn any aqueous solution: [H+] [OH-] = 10In any aqueous solution: [H+] [OH-] = 10-14-14

MM22

In a neutral solution, In a neutral solution, [H+] =10[H+] =10-7-7 M and [OH-] =10 M and [OH-] =10-7-7 M. M.In an acidic solution if the In an acidic solution if the [H+] = 10[H+] = 10-5-5 M, then [OH-] =10 M, then [OH-] =10-9-9 M. M.In a basic solution if the In a basic solution if the [H+] =10[H+] =10-9-9 M, then [OH-] = 10 M, then [OH-] = 10-5-5 M. M.


pHpH = Negative log10 of the [H+] expressed = Negative log10 of the [H+] expressed

in moles per liter.in moles per liter.

pH of 7 is a neutral solution.pH of 7 is a neutral solution.

pH < 7 is an acidic solution.pH < 7 is an acidic solution.

PH > 7 is a basic solution.PH > 7 is a basic solution.

Most biological fluids are within the Most biological fluids are within the

pH range of 6 to 8. There are some exceptionspH range of 6 to 8. There are some exceptions

such as stomach acid with pH of 1.such as stomach acid with pH of 1.


Each pH unit represents a Each pH unit represents a tenfoldtenfold difference difference

(scale is logarithmic ), so a slight change in(scale is logarithmic ), so a slight change in

pH represents a large change in pH represents a large change in

actual [H+].actual [H+].

pH = -log [H+] or [H+] = 10 pH = -log [H+] or [H+] = 10 – pH– pH M M

pOH = -log [OH-] or [OH-] = 10 pOH = -log [OH-] or [OH-] = 10 – pOH– pOH M M

**pH + pOH = 14****pH + pOH = 14**


For Example :For Example :

If the concentration of OHIf the concentration of OH-- in an in an aqueous solution is 10aqueous solution is 10-3-3, what is the , what is the pH?pH?


pOH = -log [OH-] pH + pOH = pOH = -log [OH-] pH + pOH = 1414

pOH = -log [10pOH = -log [10-3-3] pH + 3 = ] pH + 3 = 1414

pOH = 3 pH = 14 – 3pOH = 3 pH = 14 – 3

Final Answer pH = 11Final Answer pH = 11



What is the HWhat is the H++ concentration in a concentration in a solution that has a pH of 7?solution that has a pH of 7?


Answer:Answer:

1 X 10 1 X 10 –7–7 M M



How much greater is the [HHow much greater is the [H++] in a ] in a solution solution

with pH 2 than in a solution with pH 6?with pH 2 than in a solution with pH 6?


Answer:Answer:

pH 2 = [H+] of 10pH 2 = [H+] of 10-2-2 = 1 = 1 M M

100100

pH 6 = [H+] of 10-6 =pH 6 = [H+] of 10-6 = 1 1 M M

1,000,000 1,000,000

10,000 times greater.10,000 times greater.


BuffersBuffers

By minimizing wide fluctuations in pH,By minimizing wide fluctuations in pH,

buffers help organisms maintain the buffers help organisms maintain the pH of pH of

body fluids within the narrow range body fluids within the narrow range

necessary for life (usually pH 6-8).necessary for life (usually pH 6-8).


BufferBuffer 1. Substances that prevent large1. Substances that prevent large

sudden changes in pH.sudden changes in pH.2. Are combinations of H2. Are combinations of H++ -donor and -donor and

HH++ -acceptor forms of weak acids or -acceptor forms of weak acids or bases. bases.

3. Work by accepting H3. Work by accepting H++ ions from solution ions from solution when they are in excess, and by donatingwhen they are in excess, and by donating HH++ ions to the solution when they have been ions to the solution when they have been depleted.depleted.



Bicarbonate bufferBicarbonate buffer

HH22COCO33 HCO HCO33-- + H + H++

HH++ donor H donor H++ acceptor acceptor

Weak acid Weak baseWeak acid Weak base


HCl + NaHCOHCl + NaHCO33 H H22COCO33 + + NaClNaCl

Strong acid Weak acidStrong acid Weak acid

NaOH + HNaOH + H22COCO33 NaHCO NaHCO33 + + HH22OO

Strong base Weak baseStrong base Weak base

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AQUEOUS SOLUTIONS