Question

The $pH$ of a $10^{-8}$ molar solution of $HCl$ in water is

• A. $8$
• B. between $7$ and $8$
• C. between $6$ and $7$
• D. None of these

Answer 1

Answer: (C)

$pH=-\log \left[H^{+}\right]=-\log 10^{-8}=8$

It is not possible for acid, so it is $\left[H^{+}\right],$ the $\left[H^{+}\right]$ of water is also added. Total $\left[H^{+}\right]$ in solution

$=\left[H^{+}\right]$ of $HCl+\left[H^{+}\right]$ of water

$=\left(1\times 10^{-8}+1\times 10^{-7}\right)M$

$=(1+10)\times 10^{-8}=11\times 10^{-8}M$

$\therefore pH=-\log \left[H^{+}\right]$

$=-\log 11\times 10^{-8}$

$=-\log 11+8\log 10$

$=-1.0414+8=6.9586$