Gas Stoichiometry: Molar Volume Conversions & Practice Problems

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 • OR

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! • What is the volume of 77.0 g nitrogen dioxide gas at STP? 37.5 L NO2 • At STP, 3 L of chlorine is produced during a chemical reaction. What is the mass of this gas? 9 g Cl2

Gas Stoichiometry Moles  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 • Use ideal gas law to find moles • Use stoichiometry to convert moles to final answer • If given volume of gas – STP 1. Use molar volume (22.4 L/mol) to find moles 2. Use stoichiometry to convert moles to final answer

Gas Stoichiometry • If you need the volume of gas – Non-STP • Use stoichiometry to convert to moles of needed gas • Use the ideal gas law to find volume • If you need the volume of gas – STP • Use stoichiometry to convert to moles of needed gas • Use the molar volume (22.4L/mol) to find volume

Gas Stoichiometry Problem How many liters of CO2form when 15.25 g of C2H6 combusts at STP? 2C2H6 + 7O2 6H2O + 4CO2 Need: liters Use stoich to calculate moles of CO2 Use the molar volume to find liters = 1.014 mol CO2 1 mol C2H6 30.08 g C2H6 15.25 g C2H6 1 4 mol CO2 2 mol C2H6 Use the molar volume to find liters.

Gas Stoichiometry Problem How many liters of CO2form when 15.25 g of C2H6 combusts at STP? 2C2H6 + 7O2 6H2O + 4CO2 22.4 L CO2 1 mol CO2 1.014 mol CO2 1 = 22.71 L CO2 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Gas Stoichiometry Problem What volume of CO2 forms from 5.25 g of CaCO3 decomposing at 103 kPa & 25ºC? CaCO3 CaO + CO2 Need: liters Use stoich and calculate moles of CO2., Use the ideal gas law. 1 mol CaCO3 100.09 g CaCO3 1 mol CO2 1 mol CaCO3 5.25 g CaCO3 1 = 0.0525mol CO2 Plug this into the Ideal Gas Law to find liters.

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 Latm/molK WORK: Convert units first! Then apply PV = nRT Ans: 1.26L CO2

Gas Stoichiometry Problem How many grams of Al2O3 are synthesized from 15.0 L of O2 at 97.3 kPa & 21°C? Al + O2 Al2O3 Given: liters Use Ideal Gas Law and calculate moles of O2., Use stoich to find grams of Al2O3 GIVEN: P = 97.3 kPa V = 15.0 L n = ? T = 21°C R = 0.0821 Latm/molK WORK: Convert units first! Then apply PV = nRT NEXT  use stoich to find grams

Gas Stoichiometry Problem How many grams of Al2O3 are formed from 15.0 L of O2 at 97.3 kPa & 21°C? 4 Al + 3 O2 2 Al2O3 Use stoich to convert moles of O2 to grams Al2O3. 0.597 mol O2 1 2 molAl2O3 3 mol O2 101.96 g Al2O3 1 mol Al2O3 = 40.6 g 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

Gas Stoichiometry: Molar Volume Conversions & Practice Problems