Question

Which of the following solution will have a pH exactly equal to 8?

• A. ${10}^{-8}MHClsolutionat{25}^{0}C$
• B. $2\times {10}^{-6}MBa(OH{)}_{2}solutionat{25}^{0}C$
• C. ${10}^{-5}MNaOHsolutionat{25}^{0}C$
• D. None of the above

Answer 1

The correct option is D None of the above

(a)$\left[H^{+}\right]$ from $HCl$ = ${10}^{-8}M$
$\left[H^{+}\right]$ from $H_{2}O$ = ${10}^{-7}M$
$total\left[H^{+}\right]=({10}^{-8}+{10}^{-7})=1.1\times {10}^{-7}M$
we know that $pH=-lo{g}_{10}\left[H^{+}\right]$
$pH=-lo{g}_{10}(1.1\times {10}^{-7})=7-0.0414=6.9$
(b)$\left[O{H}^{-}\right]=4\times {10}^{-6}M\Rightarrow \left[H^{+}\right]\left[O{H}^{-}\right]={10}^{-14}\Rightarrow \left[H^{+}\right]=2.5\times {10}^{-9}M\Rightarrow pH=-lo{g}_{10}\left[H^{+}\right]=-lo{g}_{10}(2.5\times {10}^{-9})=9-lo{g}_{10}\left(2.5\right)=8.602$
(c) NaOH is a strong base
$\left[O{H}^{-}\right]={10}^{-5}\Rightarrow \left[H^{+}\right]\left[O{H}^{-}\right]={10}^{-14}\Rightarrow \left[H^{+}\right]={10}^{-9}M$
we know that $pH=-lo{g}_{10}\left[H^{+}\right]$
$pH=-lo{g}_{10}\left({10}^{-9}\right)=9$