(a) Since the dotted plot is parallel to P-axis, it tells that value of $\frac{PV}{T} (= nR)$ remains constant even when P is changed.(b) Curve at temperature $T_1$ is more close to the dotted plot than the curve at the temperature $T_2$. Since the behaviour of a real gas approaches the perfect gas behaviour, as the temperature is increased therefore $T_1 > T_2.$(c) If the amount of gas under consideration is 1 mole, then the value of PV/T, where the curves meet PV/T-axis will be $R (= 8.31\,\mathrm{J}\,\mathrm{mol}^{-1}\,\mathrm{K}^{-1}).$ For oxygen, molecular mass is 32.0 g i.e. $32.0 \times 10^{-3}\,\mathrm{kg}$. Since mass of oxygen gas under considerations is $1.0 \times 10^{-3}$ kg, the value of PV/T, where the curves meet the PV/T-axis is given by $\frac{PV}{T} = \frac{8.31}{32.0 \times 10^{-3}} \times 1.0 \times 10^{-3}$$= 0.26\,\mathrm{J}\,\mathrm{K}^{-1}$(d) If we obtained similar plots for $1.0 \times 10^{-3}$ kg of hydrogen, the value of PV/T, where the curves meet the y-axis will not be the same.Since molecular mass of hydrogen is $2.02 \times 10^{-3}$kg, the mass of hydrogen that will yield same value of PV/T i.e.,$0.26\,\mathrm{J}\,\mathrm{K}^{-1}$ will be$\frac{2.02 \times 10^{-3}}{8.31} \times 0.26$$= 6.32 \times 10^{-5}\,\mathrm{kg}$