13.3 Figure 13.8 shows a plot of PV/T versus P for 1.00×10-3 kg of oxygen gas at two different temperatures.

 

Fig. 13.8
(a) What does the dotted plot signify?
(b) Which is true: T1 > T2 or T1 < T2?
(c) What is the value of PV/T where the curves meet on the y-axis?
(d) If we obtained similar plots for 1.00×10-3 kg of hydrogen, would we get the same value of PV/T at the point where the curves meet on the y-axis? If not, what mass of hydrogen yields the same value of PV/T (for the low-pressure and high-temperature region of the plot)? (Molecular mass of H2 = 2.02 u, of O2 = 32.0 u, R = 8.31 J mo1–1 K–1.)
(a) The ideal behaviour of gas is shown by the dotted plot in the graph i.e., the ratio PVT equal to nR (n is the number of moles and R is the universal gas constant) is a constant quantity. It does not depend on the pressure of the gas.
(b) The dotted plot in the graph represents an ideal gas. A gas behaves more like an ideal gas at a higher temperature and lower pressure. The curve of the gas at temperature T1 is closer to the dotted plot than the curve of the gas at temperature T2. Therefore, T1 > T2 is true for the given plot.
(c) The ideal gas equation is given by:
PV = nRT 
PVT=nR
So the value of the ratio PV/T, where the two curves meet, is nR.
(d)
Molecular mass of O2 = 32.0 g
Mass of oxygen = 1 × 10-3kg = 1 g
No.of moles of the O2132 mol
PVT=nR=132x8.314 = 0.26 J k-1
Therefore, the value of the ratio PV/T, where the curves meet on the y-axis, is 0.26 J K-1.
If we obtain similar plots for 1.00 × 10-3 kg of hydrogen, then we will not get the same value of PV/T at the point where the curves meet the y-axis. This is because the molecular mass of hydrogen (2.02 u) is different from that of oxygen (32.0 u).
Let m is the mass of H2 having the same value of PV/T where the two curves meet.
Molecular mass (M) of H2 = 2.02 u
No. of moles of H2n=mM
PVT=nR at a constant temperature.
m=PVT×MR=0.26×2.028.31 = 6.3 × 10-2 g = 6.3 × 10-5kg
Hence, 6.3 × 10-5kg of H2 will yield the same value of PV/T.