Question

The $pH$ of a $10^{-10}M-NaOH$ solution is

• A. 10
• B. 7.01
• C. 6.99
• D. 4

Answer 1

Answer: (B)

When the solution is very dilute, the concentration of $O{H}^{-}$ produced from water cannot be neglected. Hence,
$\left[O{H}^{-}\right]=10^{-10}+10^{-7}$
(obtained from water)
$=10^{-7}(0.001+1)$
$=1.001\times 10^{-7}$
$\therefore pOH=-\log \left[O{H}^{-}\right]$
$=-\log \left(1.001\times 10^{-7}\right)$
$=7-0.01=6.99$
$\therefore pH=14-pOH$
$=14-6.99=7.01$