Question

If the hydrogen ion concentration in an acid is $10^{-8} \,mol / L$ then the $pH$

• A. 9
• B. 7
• C. < 7
• D. > 8

Answer 1

Answer: (C)

$pH =-\log \left[ H ^{+}\right] $
$ H ^{+}=10^{-8} \,mol / L $
So, in aqueous solution, in $H ^{+}$ion concentration of acid,
$H ^{+}$ion concentration of water is also added.
Hence, total $\left[ H ^{+}\right]$of acid
$=\left(10^{-8}+10^{-7}\right) \,mol / L $
$=10^{-8}(1+10)$
$=11 \times 10^{-8} \,mol / L$
Therefore, $pH =-\log \left[11 \times 10^{-8}\right]$
$=-\log 11+8 \log 10 $
$=-1.0414+8 $
$pH =6.9586$