Question

What is the resultant $pH$ of the solution if equal volumes of two solutions with $pH=5$ and $pH=9$ are mixed at $25{}^{o}C$?

• A. $7$
• B. $6.5$
• C. $5.5$
• D. $8.5$

Answer 1

The correct option is A $7$

Let us take the case of $1\text{L}$ solution.
$pH=5$
$-log\left[H^{+}\right]=5$
$\left[H^{+}\right]={10}^{-5}$
$pH=9$
$pOH=14-9=5$
$-log\left[O{H}^{-}\right]=5$
$\left[O{H}^{-}\right]={10}^{-5}$
Equal volume and concentration of $H^{+}$ and $O{H}^{-}$ upon mixing will give a neutral solution at $25{}^{o}C$ with $pH=7$.