Question

What is the pH of a solution prepared by mixing 30.00 mL of 0.10 MCH3CO2H with 30.00 mL of 0.030 MCH3CO2K? Assume that the volume of the solutions are additive and that Ka=1.8×10−5 for CH3CO2H.

Answer 1

Answer : The  pH of the solution is, 4.2

Explanation : Given,

$K_a=1.8\times 10^{-5}$

Concentration of $CH_3CO_2H$ = 0.10 M

Concentration of $CH_3CO_2K$ = 0.030 M

First we have to calculate the value of $pK_a$.

The expression used for the calculation of $pK_a$ is,

$pK_a=-\log (K_a)$

Now put the value of $K_a$ in this expression, we get:

$pK_a=-\log (1.8\times 10^{-5})$

$pK_a=5-\log (1.8)$

$pK_a=4.7$

Now we have to calculate the pH of buffer.

Using Henderson Hesselbach equation :

$pH=pK_a+\log \frac{[Salt]}{[Acid]}$

$pH=pK_a+\log \frac{[CH_3CO_2K]}{[CH_3CO_2H]}$

Now put all the given values in this expression, we get:

$pH=4.7+\log (\frac{0.030}{0.10})$

$pH=4.2$

Therefore, the pH of the solution is, 4.2