Flexi Says: A chemical reaction, which produces gas, is performed. The produced gas is then collected and its mass and volume are determined. The molar mass of the unknown gas can be found using the ideal gas law, provided the temperature and pressure of the gas are also known. Sample Problem: Molar Mass and the Ideal Gas Law A certain reaction occurs, producing an oxide of nitrogen as a gas. The gas has a mass of 1.211 g and occupies a volume of 677 mL. The temperature in the laboratory is 23°C and the air pressure is 0.987 atm. Calculate the molar mass of the gas and deduce its formula. Assume the gas is ideal. Step 1: List the known quantities and plan the problem. Known mass=1.211 g V=677 ml=0.677 L T=23∘C=296 K P=0.987 atm R=0.08206 L⋅atm/K⋅mol Unknown n=? mol molar mass=? g/mol First the ideal gas law will be used to solve for the moles of unknown gas (n). Then the mass of the gas divided by the moles will give the molar mass. Step 2: Solve. n=PV/RT=0.987 atm×0.677 L/0.08206 L⋅atm/K⋅mol×296 K=0.0275 mol Now divide g by mol to get the molar mass. molar mass=1.211 g/0.0275 mol=44.0 g/mol Since N has a molar mass of 14 g/mol and O has a molar mass of 16 g/mol, the formula N2O would produce the correct molar mass.
