Question

The pH value of an acid is $5$ and its concentration is 1 M. What is the value of $K_a$ for the acid?

• A. $10^{-7}$
• B. $10^{-5}$
• C. $10^{-10}$
• D. $10^{-8}$

Answer 1

Answer: (C)

By using $pH=-\log \sqrt{K_a\cdot C}$
$5=-\log \sqrt{K_a\times 1}=-\frac{1}{2}\log K_a$
$\log K_a=-10$
$K_a=10^{-10}$
Hence, dissociation constant $(K_a)=10^{-10}$