Gas Stoichiometry

Molar Volume of Gases

• The volume occupied by one mole of a gas at STP (standard temperature and pressure)

– Equal to 22.4 L / mol

– Can be used as a conversion factor

– ORgas 1mol

L 22.4

L 22.4

gas mol 1

Sample Problem 1

• A chemical reaction produces 0.0680 mol of oxygen gas. What volume in liters is occupied by this gas sample at STP?

x = 1.52 L O2

Sample Problem 2

• A chemical reaction produced 98.0 mL of sulfur dioxide gas, SO2, at STP. What was the mass of the gas produced?

• Thought process: mL L mol g

98.0 mL = 0.098 L

x x = 0.280 g SO2

PRACTICE: You Try It!

1. What is the volume of 77.0 g nitrogen dioxide gas at STP? 37.5 L NO2

2. At STP, 3 L of chlorine is produced during a chemical reaction. What is the mass of this gas? 9 g Cl2

Gas StoichiometryMoles Liters of a Gas:

– STP Conditions• use 22.4 L/mol and stoichiometry

– Non-STP Conditions• use ideal gas law and stoichiometry

General rule: FIND MOLES FIRST!– If given volume of gas – Non-STP

1. Use ideal gas law to find moles 2. Use stoichiometry to convert moles to final answer

– If given volume of gas – STP 1. Use molar volume (22.4 L/mol) to find moles2. Use stoichiometry to convert moles to final answer

Gas Stoichiometry– If you need the volume of gas – Non-STP

1. Use stoichiometry to convert to moles of needed gas2. Use the ideal gas law to find volume

– If you need the volume of gas – STP

1. Use stoichiometry to convert to moles of needed gas2. Use the molar volume (22.4L/mol) to find volume

1 mol C2H6

30.08 g C2H6

Gas Stoichiometry ProblemHow many liters of CO2 form when 15.25 g of

C2H6 combusts at STP?

15.25 g C2H6

1

= 1.014 mol CO2

2C2H6 + 7O2 6H2O + 4CO2

4 mol CO2

2 mol C2H6

Need: liters 1. Use stoich to calculate moles of CO2

2. Use the molar volume to find liters

Use the molar volume to find liters.

22.4 L CO2

1 mol CO2

Gas Stoichiometry ProblemHow many liters of CO2 form when 15.25 g of

C2H6 combusts at STP?

1.014 mol CO2

1

= 22.71 L CO2

2C2H6 + 7O2 6H2O + 4CO2

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

1 mol CaCO3

100.09 g CaCO3

Gas Stoichiometry ProblemWhat volume of CO2 forms from 5.25 g of CaCO3 decomposing at 103 kPa & 25ºC?

5.25 g CaCO3

1

= 0.0525 mol CO2

CaCO3 CaO + CO2

1 mol CO2

1 mol CaCO3

Need: liters 1. Use stoich and calculate moles of CO2., 2. Use the ideal gas law.

Plug this into the Ideal Gas Law to find liters.

WORK:

Convert units first!

Then apply PV = nRT

Ans: 1.26 L CO2

Gas Stoichiometry Problem• What volume of CO2 forms from 5.25 g of

CaCO3 at 103 kPa & 25ºC?

GIVEN: P = 103 kPa V = ? n = 1.26 mol T = 25°C R = 0.0821 Latm/molK

WORK:

Convert units first!

Then apply PV = nRT

Gas Stoichiometry ProblemHow many grams of Al2O3 are synthesized from 15.0 L of O2

at 97.3 kPa & 21°C?

GIVEN:P = 97.3 kPaV = 15.0 Ln = ?T = 21°C R = 0.0821 Latm/molK

Al + O2 Al2O3 Given: liters

1. Use Ideal Gas Law and calculate moles of O2.,

2. Use stoich to find grams of Al2O3

NEXT use stoich to find grams

2 mol Al2O3

3 mol O2

Gas Stoichiometry Problem How many grams of Al2O3 are formed

from 15.0 L of O2 at 97.3 kPa & 21°C?

0.597 mol O2

1

= 40.6 g Al2O3

4 Al + 3 O2 2 Al2O3

101.96 g Al2O3

1 mol Al2O3

Use stoich to convert moles of O2 to grams Al2O3.

You Try It!!

• Find the volume of hydrogen gas made when 38.2 g zinc reacts with hydrochloric acid at a pressure of 107.3 kPa and a temperature of 88oC.

• HINT: – Use stoich to find moles of H2 first, then use the

ideal gas law.

Zn (s) + 2 HCl (aq) ZnCl2(aq) + H2(g)

ANSWER: 16.3 L H2

You Try It!!

• What mass of solid magnesium is required to react with 250 mL carbon dioxide at 1.5 atm and 77oC to produce solid magnesium oxide and solid carbon?

• HINT: – Use ideal gas law to find moles of CO2, then use

stoich to find grams of Mg.

ANSWER: 0.63 g Mg