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Stoichiometry

Chapter 9

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Stoichiometry

• Quantitative relationship between two substances

• Composition stoichiometry: mass relationships of elements in compounds

• Reaction stoichiometry: mass relationships between reactants and products

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Mole ratio

• Used in all reaction stoichiometry problems

• Gets us from given to unknown• Conversion factor

– Relates amounts in moles for any two substances in a reaction

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• CH4(g) + 2O2(g) 2H2O(g) + CO2(g)

(g)O mol 2

(g)CH mol 1

2

4

O(g)H mol 2

(g)CH mol 1

2

4

(g)CO mol 1

(g)CH mol 1

2

4

(g)CH mol 1

(g)O mol 2

4

2

(g)CH mol 1

O(g)H mol 2

4

2

(g)CH mol 1

(g)CO mol 1

4

2

O(g)H mol 2

(g)O mol 2

2

2

(g)O mol 2

O(g)H mol 2

2

2

(g)CO mol 1

(g)O mol 2

2

2

(g)O mol 2

(g)CO mol 1

2

2

(g)CO mol 1

O(g)H mol 2

2

2

O(g)H mol 2

(g)CO mol 1

2

2

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Mole ratios

• Are exact• Do not limit the number of

significant figures

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Molar mass

• Used in many stoichiometry problems

• Mass in grams of one mole of a substance

OH g 02.18

OH mol 1

2

2

OH mol 1

OH g 8.021

2

2

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Moles to moles

• Amount of given substance in moles

• To

• Amount of unknown substance in molesGiven amount in moles

Moles of given from equation

Moles of unknown from equation

(mole ratio)

= unknown amount in moles

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Moles to mass• Given in moles

• To

• Unknown in moles• To

• Unknown in grams

Given amount in moles

Moles of given from equation

Moles of unknown from equation

(mole ratio)

1 mole of unknown

Grams of unknown

(molar mass)

= unknown amount in grams

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Mass to moles• Given in grams

• To

• Given in moles• To

• Unknown in moles

Given amount in grams

Grams of given

1 mole of given

(mole ratio)

Moles of given from equation

Moles of unknown from equation

(molar mass)

= unknown amount in moles

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Mass to mass• Given in grams

• To• Given in moles

• To• Unknown in moles

• To• Unknown in grams

Given amount in grams

Grams of given

1 mole of given

(mole ratio)

Moles of given from equation

Moles of unknown from equation

(molar mass)

= unknown amount in grams

1 mole of unknown

Grams of unknown

(molar mass)

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Carry units through

• All the way through the problem• g and mol• Elements or compounds

– This saves a lot of confusion

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Moles to moles - example

• The elements lithium and oxygen react to form lithium oxide, Li2O. How many moles of lithium oxide will form if 2.0 mol of lithium react?

• Li + O2 Li2O

• 4Li + O2 2Li2O

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You try

• The disinfectant hydrogen peroxide H2O2 decomposes to form water and oxygen gas. How many moles of O2 will result from the decomposition of 5.0 mol of hydrogen peroxide?

• H2O2 H2O + O2

• 2H2O2 2H2O + O2

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Moles to mass

• When sodium azide is activated in an automobile airbag, nitrogen gas and sodium are produced according to the equation:

• 2NaN3(s) 2Na(s) + 3N2(g)

• If 0.500 mol of NaN3 react, what mass in grams of nitrogen would result?

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You try

• Coal can be converted to methane gas by a process called coal gasification. The equation for the reaction is:

• 2C(s) + 2H2O(l) CH4(g) + CO2(g)

• What mass in grams of carbon is required to react with water to form 1.00 mol CH4?

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You try

• Using the previous reaction, what mass in grams of water is required to produce 1.00 mol CH4?

• 2C(s) + 2H2O(l) CH4(g) + CO2(g)

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Mass to moles

• Chlorine gas can be produced commercially by passing an electric current through a concentrated solution of sodium chloride.

• 2NaCl(aq) + 2H2O(l) 2NaOH(aq) + Cl2(g) + H2(g)

• If the solution contains 250 g of NaCl, how many moles of Cl2 can be produced?

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You try

• Using the previous reaction, how many moles of H2 can be produced from 250 g of NaCl?

• 2NaCl(aq) + 2H2O(l) 2NaOH(aq) + Cl2(g) + H2(g)

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Mass to mass

• Sodium peroxide reacts vigorously with water to produce sodium hydroxide and oxygen.

• 2Na2O2(s) + 2H2O(l) 4NaOH(aq) + O2(g)

• What mass in grams of O2 is produced when 50.0 g of Na2O2 react?

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You try

• Using the previous reaction, what mass in grams of water is needed to react with 50.0 g of Na2O2?

• 2Na2O2(s) + 2H2O(l) 4NaOH(aq) + O2(g)

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You try

• Milk of magnesia, a suspension of Mg(OH)2 in water, reacts with stomach acid, HCl.

• Mg(OH)2(s) + 2HCl(aq) 2H2O(l) + MgCl2(aq)

• What mass in grams of HCl is required to react with 3.00 g of Mg(OH)2?

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Limiting reactant

• AKA limiting reagent• The reactant that runs out first in a

reaction• Limits the amounts of other

reactants that will be used– And the amounts of products that will

be produced.

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Examples

• In a game of musical chairs, what is the limiting reactant?– Number of chairs

• If you have 8 hot dog buns and 10 hot dogs, what is the limiting reactant?– Number of buns

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Excess reactant

• Not completely used up in a reaction

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Example

• Methanol is synthesized according to the following equation. If 500 mol of CO and 750 mol of H2 are present, which is the limiting reactant? How many moles of methanol are produced?

• CO(g) + 2H2(g) CH3OH

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You try

• Zinc citrate is synthesized according the following reaction. If there are 6 mol of ZnCO3 and 10 mol of C6H8O7, which is the limiting reactant? How many moles of Zn3(C6H5O7)2 will be produced?

• 3ZnCO3(s) + 2C6H8O7(aq) Zn3(C6H5O7)2(aq) + 3H2O(l) + 3CO2(g)

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Example

• Aspirin is synthesized by the reaction of salicylic acid with acetic anhydride. When 20.0 g of C7H6O3 and 20.0g of C4H6O3 react, which is the limiting reagent? What mass in grams of aspirin is formed?

• 2C7H6O3 + C4H6O3 2C9H8O4 + H2O

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You try• The unbalanced equation for the

reaction of calcium phosphate with sulfuric acid is as follows:

• Ca3(PO4)2 + H2SO4 CaSO4 + H3PO4

• If 250 g of Ca3(PO4)2 react with 3 mol of H2SO4, will 3 mol of CaSO4 be formed? Why or why not?

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Theoretical yield

• The maximum amount of a product that can be produced from a given amount of reactant.

• “in theory” how much you can get

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Actual yield

• Always less than theoretical yield• The measured amount of product• What you actually get

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Percent yield

• Shows efficiency of a reaction• Ratio of actual yield to theoretical

yield

yield ltheoretica

yield actualyieldpercent %)get (to 100

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You try

• The calculated theoretical yield for a reaction is 87.2 g.

• The measured actual yield is 82.3 g.

• What is the percent yield?

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Example

• Huge quantities of sulfur dioxide are produced from zinc sulfide by means of the following reaction. If the typical yield is 86.78%, how many grams of SO2 should be expected if 4897 g of ZnS are used?

• 2ZnS + 3O2 2ZnO + 2SO2