Question

How many moles of NaOH must be removed from one litre of aqueous solution to change its pH from 12 to 11?

• A. 0.009
• B. 0.01
• C. 0.02
• D. 0.1

Answer 1

The correct option is A 0.009

$$\begin{array}{cc}\text{Initial}& \text{Final}\\ pH=12& pH=11\\ \left[H^{+}\right]={10}^{-12}M& \left[H^{+}\right]={10}^{-11}M\\ \left[O{H}^{-}\right]={10}^{-2}M& \left[O{H}^{-}\right]={10}^{-3}M\end{array}$$
Initial number of moles of $O{H}^{-}={10}^{-2}$
Final number of moles of $O{H}^{-}={10}^{-3}$
So, number of moles of $O{H}^{-}$ removed = [0.01 - 0.001]
= 0.009