Stoichiometry Goals: 1.Perform stoichiometry calculations. 2.Understand the meaning of limiting reactant. 3.Calculate theoretical and percent yields of

StoichiometryStoichiometry

Goals:1. Perform stoichiometry calculations.2. Understand the meaning of limiting

reactant.3. Calculate theoretical and percent yields of

a chemical reaction.4. Use stoichiometry to analyze a mixture of

compounds or to determine the formula of a compound.

5. Define and use molarity in solution stoichiometry.

6. Perform calculations for pH and titration problems.

StoichiometryStoichiometry: study of the quantitative relations between amounts of reactants and products.

What is STOICHIOMETRY?What is STOICHIOMETRY?

The study of the quantitative aspects of chemical reactions.

It rests on the principle of the _________ ________________.

2 Al(s) + 3 Br2(liq) ------> Al2Br6(s)You must always begin with a balanced

equation before carrying out a stoichiometry calculation.

Write a Chemical EquationWrite a Chemical Equation

• Phosphine, PH3 (g), combusts in oxygen gas to form gaseous water and solid tetraphosphorus decoxide.

Always check (and REcheck) the balancing

Information from a Balanced Information from a Balanced EquationEquation

Equation: 2 H2 (g) + O2 (g) 2 H2O (l)Molecules: 2 molecules H2 + 1 molecule O2 2 molecules H2O

Mass (amu): 4.0 amu H2 + 32 amu O2 36.0 amu H2O

Amount (mol):

Mass (g):

General Plan for Stoichiometry General Plan for Stoichiometry CalculationsCalculations

Students should become familiar with stoichiometry calculations.

Mole relationships in Chemical Mole relationships in Chemical EquationsEquations

• Stoichiometric factor – relates the amounts of any two substances involved in a chemical reaction, on a mole basis.

C3H8 + O2 CO2 + H2O

5 mol of O2 are required to burn ____ mol of C3H8

____ mol of H2O are produced for every 1 mol of C3H8 burned

____ mol of CO2 are produced for every 1 mol of C3H8 burned

propane5 3 4

If 454 g of NHIf 454 g of NH44NONO33 decomposes, how much N decomposes, how much N22O O and Hand H22O are formed? What is the theoretical O are formed? What is the theoretical

yield of products?yield of products?

STEP 1 STEP 1 Write the balanced chemical equation.

STEP 2STEP 2 Convert mass of reactant to moles of reactant.(454 g) --> moles

STEP 3STEP 3 Convert moles reactant to moles product. A. Relate moles using coefficients, write a STOICHIOMETRIC FACTOR.

STEP 3STEP 3 B) Multiply moles of reactant by the stoichiometric

factor.

STEP 4STEP 4 Convert moles product to mass product = Convert moles product to mass product = ___________________ YIELD___________________ YIELD

STEP 5STEP 5 How much N2O is formed?

Total mass of reactants = total mass of

products

STEP 6STEP 6 Calculate the percent yield.

If you isolated only 131 g of N2O, what is the

percent yield?

% yield =Actual yield

Theoretical yield* 100

Students should become familiar with % yield calculations.

What is a Limiting Reactant?What is a Limiting Reactant?

• In a given reaction, there is not enough of one reagent to use up the other reagent completely.

• The reagent in short supply _______ the quantity of product that can be formed.

Which is the Limiting Which is the Limiting Reactant?Reactant?

• Limiting reactant is

• Excess reactant is

2 NO (g) + O2 (g) - - -> 2 NO2 (g)

Reactantseactants ProductsProducts

• How many grams of CO2 are produced from 50 g of propane? (theoretical yield)

• How much O2 is required to completely burn the 50 g of propane?

• How much H2O is formed?

• If the actual yield of CO2 was 145 g. What is the % yield of the reaction?

C3H8 + O2 CO2 + H2O

50 g C3H8 (1 mol/ 44 g) = 1.14 mol C3H8 * (4 mol H2O / 1 mol C3H8)

• If you begin with 99.5 g of C3H8 and 211 g of O2, which one is the limiting reactant?

• Assuming all the limiting reactant has reacted, how much CO2 will be formed?

C3H8 + O2 CO2 + H2O

Limiting ReactantLimiting ReactantReact solid Zn with 0.100 mol HCl

(aq)

Zn + 2 HCl ---> ZnCl2 + H2

1 2 3

(See CD Screen 4.8)(See CD Screen 4.8)

Rxn 1: Balloon inflates fully, some Zn leftRxn 1: Balloon inflates fully, some Zn left

* More than enough Zn to use up the 0.100 mol * More than enough Zn to use up the 0.100 mol

HClHCl

Rxn 2: Balloon inflates fully, no Zn leftRxn 2: Balloon inflates fully, no Zn left* Right amount of each (HCl and Zn)* Right amount of each (HCl and Zn)

Rxn 3: Balloon does not inflate fully, no Zn left.Rxn 3: Balloon does not inflate fully, no Zn left.* Not enough Zn to use up 0.100 mol HCl* Not enough Zn to use up 0.100 mol HCl

Zn + 2 HCl ---> ZnClZn + 2 HCl ---> ZnCl22 + + HH22

0.10 mol of HCl, need ? mol Zn.

0.10 mol HCl 1 mol Zn 2 mol HCl

= 0.050 mol Zn

Rxn 1Rxn 1 Rxn 2Rxn 2 Rxn 3Rxn 3mass Zn (g)mass Zn (g) 7.007.00 3.273.27 1.311.31mol Znmol Zn 0.1070.107 0.0500.050 0.0200.020mol HClmol HCl 0.1000.100 0.1000.100 0.1000.100mol HCl/mol Znmol HCl/mol ZnLim ReactantLim Reactant

1 2 3

Mix 5.40 g of Al with 8.10 g of ClMix 5.40 g of Al with 8.10 g of Cl22. . What mass of AlWhat mass of Al22ClCl66 can form? can form?

Mass reactant

StoichiometricfactorMoles

reactantMoles product

Mass product

___Al + ____Cl___Al + ____Cl22 ---> Al ---> Al22ClCl66


STEP 1STEP 1 FIND THE LIMITING REAGENT. FIND THE LIMITING REAGENT. CCompare actual mole ratio of reactants to theoretical mole ratio.

___ Al + ___Cl___ Al + ___Cl22 ---> Al ---> Al22ClCl66

Students should become familiar with calculations using the concept of limiting

reagent.


Calculate moles of each reactant.

Find the mole ratio of reactants:


Limiting reactant = _____BASE ALL CALCULATIONS on LR ____

moles____

moles Al2Cl6

mass____

mass Al2Cl6

__ Al + ___ Cl__ Al + ___ Cl22 ---> Al ---> Al22ClCl66

Write conversion factor:


STEP 2STEP 2 CALCULATE THE MASS OF THE PRODUCT.Calculate moles of Al2Cl6 expected based on LR.

__ Al + __ Cl__ Al + __ Cl22 ---> Al ---> Al22ClCl66

Mix 5.40 g of Al with 8.10 g of Cl2.How much of each reactant will remain

when reaction is complete?

__ Al + __ Cl__ Al + __ Cl22 ---> Al ---> Al22ClCl66

Chemical AnalysisChemical Analysis• An impure sample of the mineral thenardite

contains Na2SO4. A mass of mineral sample weights 0.123 g. The Na2SO4 in the sample is converted to insoluble BaSO4 by adding BaCl2. The recovered mass of BaSO4 is 0.177 g. What is the mass percent of Na2SO4 in the mineral?

Chemical AnalysisChemical Analysis

General Plan for Stoichiometry General Plan for Stoichiometry CalculationsCalculations

Chemical AnalysisChemical AnalysisBalanced equation:

Grams to moles

Moles to moles

Moles to grams

Mass %

Combustion Analysis of Combustion Analysis of HydrocarbonsHydrocarbons

Active Figure 4.9Active Figure 4.9

Procedure for Calculating Procedure for Calculating Empirical FormulaEmpirical Formula

Gramsof eachelement

UseMolarmass

Molesof eachelemen

t

EmpiricalFormula

Calculatemole ratio

The central part of the calculation is determining the number of moles of each element in the compound.Remember: in the mole ratio, divide by smaller number, then multiply ‘til whole.

What is the empirical formula of a hydrocarbon, CxHy, if 0.115 g burn and

produce 0.379 g of CO2 and 0.1035 g of H2O.

CxHy + some oxygen ---> 0.379 g

CO2 + 0.1035 g H2O

First, recognize that all C in COFirst, recognize that all C in CO22 and all H in and all H in

HH22O is from CO is from CxxHHyy..

1. Calculate amount (in moles) of C in CO1. Calculate amount (in moles) of C in CO22

2. Calculate amount (in moles) of H in H2. Calculate amount (in moles) of H in H22OO

3. Find ratio of mol H/mol C to find values of x 3. Find ratio of mol H/mol C to find values of x and y in Cand y in CxxHHyy..

What is the empirical formula of a hydrocarbon, CxHy, if 0.115 g burn and

produce 0.379 g of CO2 and 0.1035 g of H2O.

1. Calculate amount (moles) of C in CO2

2. Calculate amount (moles) of H in H2O

3. Ratio of mol H/mol C to find values of x and y in CxHy.

PracticePractice

4.67 Titanium (IV) oxide, TiO2, is heated in hydrogen gas to give water and a new titanium oxide, TixOy. If 1.598 g of TiO2 produces 1.438 g of TixOy, what is the formula of the new oxide?

TiO2 + H2 H2O + TixOy

1.Calculate amount of Ti:

Practice…Practice…

2. Calculate amount of O:

3. Calculate the molar ratio of O to T

How are Reactions in Solution How are Reactions in Solution Quantified?Quantified?

In solution we need to define the -

• SOLVENT the component whose

physical state is ____________ when solution forms

• SOLUTEthe other solution component

What is Molarity?What is Molarity?

The amount of solute in a solution is given by its concentration.

Molarity =

Students should become familiar with calculations using

MOLARITY.

Calculate molarity of a solution of 5.00 Calculate molarity of a solution of 5.00 gg of NiCl of NiCl22•6 •6 HH22O dissolved in enough water to make 250 O dissolved in enough water to make 250 mLmL of of

solution.solution.

STEP 1STEP 1 Calculate the number of moles of solute

STEP 2STEP 2 Calculate the molarity of the solution

How many IONS are in the How many IONS are in the Solution?Solution?

What mass of oxalic acid, H2C2O4, is required to make 250 mL of a 0.0500 M solution?

STEP 1STEP 1 Calculate moles of acid required.Calculate moles of acid required.

STEP 2STEP 2 Calculate mass of acid required.Calculate mass of acid required.

Preparing SolutionsPreparing Solutions

• Weigh out a solid solute and dissolve in a given quantity of solvent.

• Dilute a concentrated solution to give one that is __________ concentrated.

Preparing SolutionsPreparing Solutions

You have 50.0 mL of 3.0 M NaOH and You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you you want 0.50 M NaOH. What do you

do?do?

Add water to the 3.0 M solution to lower its concentration to 0.50 M

Dilute the solution!

3.0 M NaOH 0.50 M NaOH

H2O

Concentrated Dilute

M = moles/L

Notice that the amount of NaOH (moles of NaOH) present did not change.


do?do?But how much water do we add?

Amount of NaOH in original sol. = Amount of NaOH in original sol. =

Amount of NaOH in final sol. must also =Amount of NaOH in final sol. must also =

Volume of final solution =Volume of final solution =

moles of NaOH in ORIGINAL solution moles of NaOH in ORIGINAL solution = =

moles of NaOH in FINAL solutionmoles of NaOH in FINAL solution


do?do?Conclusion:Conclusion:

add ______ of wateradd ______ of water to the 50.0 mL of 3.0 M to the 50.0 mL of 3.0 M NaOH to make ________ of 0.50 M NaOH.NaOH to make ________ of 0.50 M NaOH.

A shortcut:A shortcut:

Cinitial • Vinitial = Cfinal • VfinalCinitial • Vinitial = Cfinal • Vfinal

• Principle of dilution: addition of solvent does not change the amount of solute in a solution but does change its concentration.

Solution StoichiometrySolution Stoichiometry

• Zinc reacts with acids Zinc reacts with acids to produce Hto produce H22 gas. gas.

• Have 10.0 g of ZnHave 10.0 g of Zn• What volume of 2.50 What volume of 2.50

M HCl is needed to M HCl is needed to convert the Zn convert the Zn completely?completely?

General Plan for Solution General Plan for Solution StoichiometryStoichiometry

M = moles / volumeMoles = M * volume

Zinc reacts with acids to produce HZinc reacts with acids to produce H22 gas. If gas. If you have 10.0 g of Zn, what volume of 2.50 you have 10.0 g of Zn, what volume of 2.50

M HCl is needed to convert the Zn M HCl is needed to convert the Zn completely?completely?

Stoichiometricfactor

Zinc reacts with acids to produce HZinc reacts with acids to produce H22 gas. If you gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is have 10.0 g of Zn, what volume of 2.50 M HCl is

needed to convert the Zn completely?needed to convert the Zn completely?

Step 1: Write the balanced equationStep 1: Write the balanced equation

Step 2: Calculate amount of Zn in molesStep 2: Calculate amount of Zn in moles

Step 3: Use the stoichiometric factorStep 3: Use the stoichiometric factor

Zinc reacts with acids to produce HZinc reacts with acids to produce H22 gas. If you gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is have 10.0 g of Zn, what volume of 2.50 M HCl is

needed to convert the Zn completely?needed to convert the Zn completely?

Step 4: Calculate volume of HCl req’dStep 4: Calculate volume of HCl req’d

* Use molarity (M) as conversion factor

What is pH?What is pH?• It is a concentration scale.• pH:pH: a way to express acidity – the

concentration of H+ in solution.

Acidic solution pH < 7 Neutral pH = 7 Basic solution pH > 7

Acidic solution pH < 7 Neutral pH = 7 Basic solution pH > 7

Low pH: ______ [HLow pH: ______ [H++]] High pH: ____ [HHigh pH: ____ [H++]]

The pH ScaleThe pH Scale

pH pH = log (1/ [H= log (1/ [H++]) ])

= - log [H= - log [H++]]In a neutral solution, In a neutral solution,

[H[H++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M at 25 M at 25 ooCC

pH = - log [HpH = - log [H++] = -log (1.00 x 10] = -log (1.00 x 10-7-7) ) = - [0 + (-7)] = - [0 + (-7)]

= 7= 7 See CD Screen 5.17 for a tutorial

See book Appendix A.3 for more on logs

pH and [H+]pH and [H+]

If the [H+] of soda is 1.6 x 10-3 M, the pH is ____?

pH and [H+]pH and [H+]

If the pH of Coke is 3.12, what is the [H+]?

What is a Titration?What is a Titration?

• Titration – procedure in which two reactants in solution react in the precise proportions shown by the chemical equation for the reaction.

• Buret – a calibrated instrument used in a titration. It is a graduated, long glass tube calibrated to deliver precise volumes of solution through a stopcock valve.

• Equivalence point – the point in a titration at which one reactant has been exactly consumed by addition of the other reactant.

Acid-Base TitrationAcid-Base Titration• How do we measure the concentration of

an acid in a solution?1) A measured volume of a solution of an acid

of unknown concentration is transferred to a flask.

2) A solution of a base of known concentration is added carefully from a buret until the reaction of the acid with the base is just complete.

3) Equivalence point of the titration – the point at which the acid is just neutralized. At that point, the number of moles of OH- added equals the number of moles of H+ that were in the sample of acid.

4) The equivalence point is determined with an indicator dye – a substance that changes color as the reaction is completed (litmus, phenolphthalein).

What is a Titration?What is a Titration?

Phenolphthalein indicator

1.065 g of H1.065 g of H22CC22OO44 (oxalic acid) requires 35.62 mL of (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is NaOH for titration to an equivalence point. What is

the concentration of the NaOH?the concentration of the NaOH?

Step 2:Step 2:

Step 3:Step 3:

Step 1: Step 1:

1.065 g of H1.065 g of H22CC22OO44 (oxalic acid) requires 35.62 mL of (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is NaOH for titration to an equivalence point. What is

the concentration of the NaOH?the concentration of the NaOH?

PracticePractice

What volume of 0.155 M FeCl3 contains 12.5 g FeCl3?

Apples contain malic acid, HApples contain malic acid, H22CC44HH44OO55..76.80 g of apple requires 34.56 mL of 0.663 M 76.80 g of apple requires 34.56 mL of 0.663 M

NaOH for titration. What is weight % of malic acid?NaOH for titration. What is weight % of malic acid?

__H__H22CC44HH44OO55(aq) + __NaOH(aq) ---> (aq) + __NaOH(aq) ---> __Na __Na22CC44HH44OO55(aq) + (aq) +

__H__H22O(l)O(l)

?

76.80 g of apple requires 34.56 mL of 0.663 M 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid?NaOH for titration. What is weight % of malic acid?

Step 2:Step 2:

Step 3:Step 3:

Step 1:Step 1:

76.80 g of apple requires 34.56 mL of 0.663 M 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid?NaOH for titration. What is weight % of malic acid?

Step 4:Step 4:

Step 5:Step 5:

RememberRemember

• Go over all the contents of your textbook.

• Practice with examples and with problems at the end of the chapter.

• Practice with OWL tutor.• Work on your assignment for OWL

Chapters 4 and 5.• Practice with the quiz on CD of

Chemistry Now.

Documents

Stoichiometry Goals: 1.Perform stoichiometry calculations. 2.Understand the meaning of limiting reactant. 3.Calculate theoretical and percent yields of