Molar volume (Vm) of a van der Waals gas can be calculated by expressing the van der Waals equation as a cubic equation with Vm as the variable. The ratio (in mol dm-3) of the coefficient of Vm2 to the coefficient of Vm for a gas having van der Waals constants a = 6.0 dm6 atm mol-2 and b = 0.060 dm3 mol-1 at 300 K and 300 atm is . Use: Universal gas constant (R) = 0.082 dm3 atm mol-1 K-1
Answer
-7.2 to -7
Solution
(P + aVm2 )(Vm - b) = RT gives Vm3 - (b + RTP )Vm2 + aPVm - abP = 0
Ratio = - (b + RTP ) aP = - bP + RTa
bP = 0.060 300 = 18 and RT = 0.082 300 = 24.6
Ratio = - 18 + 24.66.0 = -7.1 mol dm-3
Answer: −7.1