Key to In-Class Group Activity #5 (Chem 101, Summer 2006)
1. If P and n are constant, what will be the new V of a balloon that is initially at 1.0 L
and 0ºC if the T is increased to 25ºC?
V1/T1 = V2/T2
V2 = V1T2/T1 = (1.0 L)(298 K)/(273 K) = 1.1 L
2. A steel cylinder with a volume of 15.0 L is filled with 50.0 g of N2 gas at 25ºC. What
is the pressure of the N2 gas in the cylinder?
PV = nRT
n = (50.0 g N2)(1 mol N2/28.02 g N2) = 1.78 mol N2
T = 25C + 273 = 298 K
P = nRT/V = (1.78 mol)(0.0821 Latm/molK)(298 K)/(15.0 L) = 2.9 atm
(or P = nRT/V = (1.78 mol)(62.4 LmmHg/molK)(298 K)/(15.0 L) = 2200 mmHg
3. If a 0.300 L glass container is filled with CO2 gas at 22.0ºC and 765 mmHg, and the
mass of the CO2 in the container is measured to be 0.500 g, what is the experimentally
determined molar mass of CO2?
Since molar mass is g per one mol, we need to first solve for mol CO2.
PV = nRT
T = 22.0C + 273 = 295 K
n = PV/RT = (765 mmHg)(0.300 L)/(62.4 LmmHg/molK)(295 K) = 0.0125 mol CO2
Molar mass = 0.500 g/0.0125 mol = 40.0 g/mol
4. What is the density (g/L) of carbon dioxide gas at STP?
At STP: 1 mol CO2 = 22.4 L CO2
Molar mass CO2 = (12.01 g) + (2 x 16.00 g) = 44.01 g
d = m/V = 44.01 g/22.4 L = 1.96 g/L
5. If a scuba diving tank has a total pressure of 2400 torr, and contains 2.0 moles He and
6.0 moles O2, what is the partial pressure (in torr) of each gas in the tank?
Since the total pressure is equal to the sum of the partial pressures of He and O 2, and
the pressure of each gas is proportional to the moles of gas, then the fraction of the total
moles (this is called the mole fraction) for each gas is equal to the fraction of the total
pressure for each gas.
For He: 2.0 mol/(2.0 mol + 6.0 mol) = 1/4 (so 1/4 of the pressure comes from the He)
(1/4)(2400 torr) = 600 torr = 6.0 x 102 torr
For O2: 6.0 mol/(2.0 mol + 6.0 mol) = 3/4 (so 3/4 of the pressure comes from the O2)
(3/4)(2400 torr) = 1800 torr = 1.8 x 103 torr