Question

A weak organic acid has a K_a value of 1.2x10^-6 at 25 °C.

(a) A buffer is prepared by mixing 500 mL 0.10 M of this weak acid solution with 250 mL of 0.04 M NaOH solution, which was further diluted to a volume of 1.25 L. Calculate the pH of the buffer solution.

(b) What will be the pH of this buffer solution after further addition of 250 mL of 0.02 M NaOH solution?

Answer 1

$k_a=1.2\times 10^{-6}$

(a) V=500ml, M=0.1M ,

$P_h=-log[H^{+}]=-log(2\times 10^{-13})=13-0.301=12.7$

(b) V=250ml, M=0.02

$P_h=P_{H_a}+log\dfrac{250}{500}=12.7+0.301=13$