http://www.ck12.org Chapter 14. The Behavior of Gases
• T = 19°C = 292 K
- mass of O 2 = 3.760 g
- molar mass of O 2 = 32.00 g/mol
- R = 8.314 J/K•mol
Unknown
- V =? L
In order to use the ideal gas law, the number of moles of O 2 (n) must be found from the given mass and the molar
mass. Then, use PV = nRT to solve for the volume of the sample.
Step 2: Solve.
3 .760 g×
1 mol O 2
32 .00 g O 2
= 0 .1175 mol O 2Rearrange the ideal gas law and solve for V.
V=
nRT
P=
0 .1175 mol× 8 .314 J/K·mol×292 K
88 .4 kPa= 3 .23 L O 2
Step 3: Think about your result.
The number of moles of oxygen is far less than one mole, so the volume should be fairly small compared to molar
volume (22.4 L/mol) since the pressure and temperature are reasonably close to standard. The result has three
significant figures because of the values for T and P. Since a joule (J) = kPa•L, the units cancel correctly, leaving a
volume in liters.
Practice Problem- At what Celsius temperature does 0.815 mol of an ideal gas occupy 33.7 L at a pressure of 108.1 kPa?
- What is the pressure (in atm) in a 850. mL flask that holds 9.44 g of chlorine gas (Cl 2 ) at 11°C?
Finding Molar Mass and Density
A chemical reaction is performed that produces a gas, which is then collected. After determining the mass and
volume of the collected sample, 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 14.6: Molar Mass and the Ideal Gas Law
A certain reaction occurs, producing a gaseous oxide of nitrogen. 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