STOICHIOMETRY OF GASESChapter 11.3

1. Explain how Gay-Lussac’s law and Avogadro’s law apply to the volumes of gases in chemical reactions.

2. Use a chemical equation to specify volume ratio for gaseous reactants or products, or both.

3. Use volume ratios and the gas laws to calculate volumes, masses, or molar amounts of gaseous reactants or products.

OBJECTIVES:

Volume – Volume Calculations For gaseous reactants and products:

Coefficients can indicate volume ratios as well Only for gases

2 CO(g) + O2 (g) 2 CO2 (g)2

molecules2 molecules

1 molecules2 moles 1 moles

2 moles2 volumes 1 volumes 2 volumes

• Possible Volume Ratios:2 volumes CO2 volumes CO2

or2 volumes CO22 volumes CO

2 volumes CO1 volumes O2

or1 volumes O2

2 volumes CO

Sample Problem 1 Propane, C3H8, is a gas that is sometimes used as a fuel

for cooking and heating. The complete combustion of propane occurs according to the equation below.

(a) What will be the volume, in liters, of oxygen required for the complete combustion of 0.350 L of propane?

(b) What will be the volume of carbon dioxide produced in the reaction?Assume constant temperature and

pressure.C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)mass

molesVolume• Unlike mass : at constant temperature and pressure, volume can be determined from volume

0.350 L

• Only for GASES!!!

5

11.75 L

3

5 1.05 L

Volume-Mass and Mass-Volume Calculations

Problem type 1:gas volume A moles A moles B mass of B Problem type 2:mass A moles A moles B gas volume B

Sample Problem 2Calcium carbonate, CaCO3, also known as limestone, can be heated to produce calcium oxide (lime), an industrial chemical with a wide variety of uses. How many grams of calcium carbonate must be decomposed to produce 5.00 L of carbon dioxide at STP.

CaCO3 (s) CaO (s) + CO2 (g)

PV = nRT

mass

moles

0.223 mol

22.3 g

n =

RT RT PV

RT = (1 atm)(5.00 L)(0.0821 )

(273 K)

L*atmmol*K

= o.223 mol CO2

1

10.223 mol

x 100.09 g/mol

** find moles of CO2 using PV=nRT** Use the value you get for moles in stoichiometry.** Can only do this for gases!!!!

Sample Problem 3Tungsten, W, a metal used in light-bulb filaments, is produced industrially by the reaction of tungsten oxide with hydrogen. How many liters of hydrogen gas at 35oC and 0.0980 atm are needed to react completely with 875 g of tungsten oxide.

WO3 (s) + 3 H2 (g) W (s) + 3 H2O(g)mass

moles

875 g

÷ 231.84 g/mol

3.77 mol

3

1 11.3 mol

** find moles of H2

using stoichiometry first** Use PV=nRT to find the volume

PV = nRT

P P

V =

nRT

P = (11.3 mol)

(o.o821 )(308 K)(0.0980 atm)

L*atmmol*K

= 292 L