A gas is at its Boyle temperature. Which of the following is the best approximation for $Z$ at molar volume $V_{m}$ , retaining the lowest non-vanishing correction, for a van der Waals gas?

Answer: C

T_{B}

the second virial coefficient

B = b - a / RT

is zero, so the

1/ V_{m}

term disappears. The next term is

C / V_{m}^{2}

with

C = b^{2}

for a van der Waals gas, giving

Z \approx 1 + b^{2} / V_{m}^{2}

, a small positive deviation.