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

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

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

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

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

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

$=-1.0414+8=6.9586$