Which statement correctly describes how increasing pressure affects a gaseous equilibrium?

• A. It shifts toward the side with fewer moles of gas
• B. It shifts toward the side with more moles of gas
• C. It has no effect on equilibrium position
• D. It changes the rate but not the position

Answer: (A)

When pressure increases for a gaseous system at equilibrium, the system responds by shifting to reduce the effect of the change. It does this by moving toward the side that has fewer moles of gas, because fewer gas molecules contribute less to the pressure. This is why the statement describing a shift toward the side with fewer moles is the correct description.

A concrete example helps: consider a reaction where one mole of N2O4 ⇌ two moles of NO2. The left side has fewer gas moles, so increasing pressure pushes the equilibrium toward N2O4, lowering the total number of gas molecules and counteracting the pressure increase.

The other ideas don’t fit the situation: shifting toward more moles would not counteract the higher pressure; if both sides had the same number of moles, pressure changes wouldn’t shift the position, and claiming only the rate changes ignores that a pressure change can alter the equilibrium position when moles differ.