What is the $\mathrm{pH}$ of $\mathrm{NaOH}$ solution when $0.04\mathrm{gm}$ of it dissolved in water and made to $100\mathrm{ml}$ solution?

$\left[\mathrm{NaOH}\right]=\frac{(0.04/40)}{0.1}=0.01\mathrm{M}$

Now, sodium hydroxide is monoacidic, so

$\left[\mathrm{NaOH}\right]=\left[{\mathrm{OH}}^{-}\right]=0.01\mathrm{M}$

$⟹\mathrm{pOH}$=-\log (0.01)=2

$⟹\mathrm{pH}$=14-$\mathrm{pOH}$=14-2=12

$\mathrm{pH}$=12

Hence the correct answer (D) 12