Question

When $0.4$ g of $NaOH$ is dissolved in one litre of solution, the $pH$ of the solution is:

• A. $12$
• B. $2$
• C. $6$
• D. $10$

Answer 1

The correct option is A $12$

$NaOH\to {Na}^{+}+{OH}^{-}$

Strong base completely dissociates, therefore $\left[{OH}^{-}\right]=\frac{0.4}{40}mol/L={10}^{-2}M$

Since,

$\left[{OH}^{-}\right]\left[H^{+}\right]={10}^{-14}M$

$\left[H^{+}\right]={10}^{-12}M$

$pH=-log\left(\left[H^{+}\right]\right)$

$pH=12$

Therefore option $A$ is correct.