Question

Which of the following graph is correct for real gases other than hydrogen and helium at 0°C?

• A. (a)
• B. (b)
• C. (c)
• D. (d)

Answer 1

Answer: (D)

d

Answer 2

The compressibility factor $Z = \frac{PV}{nRT}$. For real gases (excluding H$_2$ and He) at temperatures below their Boyle temperature (like 0°C for many common gases), attractive forces dominate at low pressures, leading to $Z < 1$. At high pressures, repulsive forces dominate, leading to $Z > 1$. This results in a P-V isotherm that starts below the ideal gas curve and crosses it to lie above it at higher pressures.