Examstyle Questions: Thermodynamics and Ideal Gases

Study the graphs A, B, C, D
a) Which graph shows the variation in volume (y) of a fixed mass of ideal gas at constant pressure with absolute temperature (x)?
answer ................................
(1 Mark)
b) Which graph shows the variation in pressure (y) of a fixed mass of ideal gas at constant temperature with volume (x)?
answer ................................
(1 Mark)
(Marks available: 2)
Answer outline and marking scheme for question: 1
a) answer .............A...................
(1 Mark)
b) answer .............B...................
(1 Mark)
(Marks available: 2)

4.0 g of helium contains one mole (6.0 x 10^{23} atoms). The helium is at a presure of 1.0 x 10^{5} Pa and at a temperature of 300 K.
a) Show that one mole of helium occupies a volume of about 0.025 m^{3} under these conditions.
molar gad constant R = 8.3 J mol^{1} K^{1}
(2 Marks)
b) The gas is compressed to volume of 0.020 m^{3}. The temperature of the gas is kept constant.
Calculate the new pressure of the gas.
pressure = ..................... pA
(2 Marks)
(Marks available: 4)
a) pV = nRT (1 Mark) > V = nRT/p = 1 x 8.31 x 300 / 1.0 x 10^{5} (1 Mark) = 0.0249 m^{3}
(can work backwards or use pV = nKt)
(2 Marks)
b) 1.0 X 10^{5} X 0.025 = P_{2} X 0.020 (1 Mark) p_{2} = 1.3 x 10^{5} Pa (1 Mark) (accept 1.25 x 10^{5})
candidates can use their answers to 5 a) (ecf) (e.g. 1.2 x 10^{5} if 0.0249 used).
(2 Marks)
(Marks available: 4)

Two students attempt the same experiment to find how air pressure varies with temperature. They heat identical sealed glass flasks of air, to be considered as an ideal gas, in an oil bath. The flasks are heated from 300 K to 400 K. The pressure in flask A rises from atmospheric presure, p_{o}, as expected, but the pressure in flask B remains at p_{o }because the rubber bung is defective and air leaks out of the flask.
a) Calculate the pressure in flask A at 400 K in terms of p_{o}.
pressure = ........................
(2 Marks)
b) Calculate the fraction, f, of gas molecules in flask B compared to flask A at 400 K.
f = number of gas molecule in B at 400 K number of gas molecule in A at 400 K f = ...............................
(2 Marks)
(Marks available: 4)
a) p/T = constant / AW; p/p_{o} = T_{1}/T_{o} = 400/300 giving p = 1.33 p_{o}
(2 Marks)
b) use of n = pV/RT / n (proportional to) p / N (proportional to) p / f = (N_{B}/N_{A}) = n_{B}/n_{A} = p_{o}/p
= ¾ or 0.75 ecf from c(i)
(2 Marks)
(Marks available: 4)