Question

The pH of a $10^{-10}MNaOH$ 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)
$=10-7(0.001+1)$
$\therefore pOH=-\log \left[O{H}^{-}\right]$
$=-\log \left(1.001\times 10^{-7}\right)$
$=7-0.01=6.99$
$\therefore pH=14-pOH$
$=14-6.99=7.01$