Question

What is the pH at ${25}^{\circ }\text{C}$, if a solution which is twice as alkaline (i.e. which contains twice as many hydroxide ions) as pure water?

• A. 6.7
• B. 7.3
• C. 9.0
• D. 8.0

Answer 1

The correct option is B 7.3

For a solution to have twice alkalinity, we have
$\left[O{H}^{-}\right]=2.0\times {10}^{-7}M$
Thus $\text{pOH}=-\log \left\{\right[O{H}^{-}\left]\right\}=-\log (2.0\times {10}^{-7})=7-0.3=6.70$
and hence $\text{pH}=14-\text{pOH}=7.30$