Question

$0.4g$ of $NaOH$ present in one litre solution, then the $pH$:

• A. $12$
• B. $2$
• C. $6$
• D. $10$

Answer 1

The correct option is A $12$

Molecular mass of $NaOH$=(23+16+1)=40

No. of moles of 0.4 g of NaOH $=\frac{0.4}{40}$= 0.01 mole

0.01 mole of NaOH in 1 lit.

Hence, molarity $=\frac{mole}{volume}={10}^{-2}$M

As $NaOH$ is a strong electrolyte, it dissociates completely.

Hence, [$NaOH$]=[$O{H}^{-}$]

Now,

$pOH=-log\left[O{H}^{-}\right]=-log\left({10}^{-2}\right)=2$

Again,

$pOH+pH=14$

$\therefore pH=14-2=12$

Hence, option $A$ is correct.