**Practice Problem 2**

Calculate the rate constant for the reaction between phenolphthalein and the OH^{-}
ion if the instantaneous rate of reaction is 2.5 x 10^{-}5 mole per liter per
second when the concentration of phenolphthalein is 0.0025 *M*.

**Solution**

We start with the rate law for this reaction:

Rate = *k*(phenolphthalein)

We then substitute the known rate of reaction and the known concentration of phenolphthalein into this equation:

Solving for the rate constant gives the following result:

*k *= 0.010 s^{-1}