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MCAT Chemistry Question — Rate Law

The reaction A + B → C proceeds at a rate that varies with the initial concentration of the reactants, as follows:





Rate t=0 (M/s)


0.1 M

0.5 M

5 x 10-4


0.2 M

1.0 M

1 x 10-3


0.1 M

4.0 M

4 x 10-3


What is the rate law for the reaction above?


  1. Rate = k[A]
  2. Rate = k[B]
  3. Rate = k[A][B]
  4. Rate = k[A][B]2
Click for Explanation

To determine the rate law, you must compare trials to see how different initial reactant concentrations affect the initial rate of the reaction. Comparing trial 1 and trial 3, the initial rate in trial 3 increased by a factor of 8. Since [A] is constant and [B] increases by a factor of 8, the reaction rate is first order with respect to B.

Comparing trial 1 and trial 2, the initial rate of trial 2 increase by a factor 2. Since [B] also increases by a factor of 2 and the reaction rate is first order with respect to B, the increase in the reaction rate is due to [B] alone. Thus, the reaction is zero order with respect to A. Thus, the rate law can be described by answer choice B.

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