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School of Chemistry - UKZN
Tutorial 7 - Gases and Gas Laws
1.
The pressure outside jet plane flying at high altitude falls considerably below standard
atmospheric pressure. Therefore, air inside the cabin must be pressurised to protect the
passengers. What is the pressure in atmospheres in the cabin if the barometer reading is
688mmHg. (Ans. 0.905 atm)
Answer: Coversion of mmHg to atm use 1atm = 760 mmHg
2.
Calculate the volume (in Litres) of an ideal gas which occupies 49.80g of HCl at STP?
(Ans. 30.61 L)
Answer: nHCl = 49.80g /36.5 g/mol = 1.36 mols
At STP :
1 mol = 22.414 dm3
1.36 mol = X
X = 30.61 dm3 (L)
3.
A gas initially at 4.0L, 1.2atm and 66oC undergoes a change so that its final volume and
temp. become 1.7 L and 42oC respectively. What is the final pressure of the gas?
Assume that the no. of moles remains (constant) (Ans. 2.6 atm)
Answer: Using the general gas equation: P1V1 = P2V2
T1
Solving for P2 = P1V1T2
T2
P2 = 2.6 atm
V2
(Convert oC to K in all cases)
4.
A sample of oxygen gas initally at 0.97 atm was cooled from 21oC to -68 oC at constant
volume. What is its final pressure (in atm)? (Ans. 0.68atm)
Answer:
Using the general gas equation: P1V1 = P2V2
T1
At constant volume : P1 = P2
T1
T2
T2
solving for P2 = 0.68 atm
2
5.
What is the density (g/L) of uranium heaxfluoride (UF6) at 779 mmHg and 62 oC?
(Ans. 13.10g/L)
Use the equation:  (gas) =
P x MM
RT
Molar mass of UF6 = 352.02 g/mol
6.
The density of a gaseous organic compound is 3.38 g/L at 40oC and 1.97 atm. Calculate
it’s molar mass. (Ans. 44.11 g/mol)
Answer: Use this equation MM(gas) =
7.

RT
P
A sample of natural gas contains 8.24 mols of CH4(g), 0.421 mols of C2H6(g) and 0.116
mols of C3H8 (g). If the total pressure of the gas is 1.37 atm, calculate the partial
pressures of the individual gases.
(Ans. CH4(g) = 1.29 atm ; C2H6(g) = 0.0658 atm; C3H8 (g) = 0.0181atm)
Answer: Calculate total number of mole ntotal = 8.24mol + 0.421 mol + 0.116 mol = 8.777
For CH4: 8.24/8.777 = 0.94 therefor the PCH4 = 1.37 atm X 0.94 = 1.29 atm.
C2H6 (g) and C3H8 (g) are calculated in the same way
8.
A mixture of a sample of gases contains 4.46 moles of Ne, 0.74 moles of Argon and 2.15
moles of Xe. Calculate the partial pressures of the gases if the total pressure is 2.00 atm at
a certain temperature. (Ans. PNe 1.21 atm; PAr= 0.20 atm; PXe = 0.586 at m)