If the number of moles doubles and the volume doubles while the temperature remains constant, what happens to the pressure?

• A. P increases
• B. P decreases
• C. P remains the same
• D. P becomes zero

Answer: (C)

The pressure of a gas at constant temperature is governed by PV = nRT, so P is proportional to n/V. If you double the number of moles and also double the volume, the ratio becomes (2n)/(2V) = n/V, so the pressure stays the same. This relies on ideal gas behaviour; in real gases there can be deviations, but under typical conditions the pressure would remain unchanged.