For the rate law rate = k [A]^2 [B], by what factor does the rate increase if [A] is tripled and [B] is doubled?

• A. 12
• B. 9
• C. 18
• D. 6

Answer: (C)

The rate law tells us rate ∝ [A]^2 [B]. If [A] is tripled, the [A]^2 term goes to (3)^2 = 9 times larger. If [B] is doubled, the [B] term goes to 2 times larger. The overall rate increases by 9 × 2 = 18. So the rate goes up by a factor of 18.