Question

The pH of $10^{-8}M \,HCl$ solution is

• A. 8
• B. 6.9586
• C. More than 8
• D. Slightly more than 7

Answer 1

Answer: (B)

As $HCl$ solution is acidic in nature and the $pH$ of an acidic solution cannot exceed more than $7$ Thus, the answer must be $6.9586$

We can also find this answer through calculations From acid, $\left[ H ^{+}\right]=10^{-8} \,M$

But the $\left[ H ^{+}\right]$ ions from water, i.e., $\left[ H ^{+}\right]=10^{-7}\, M$

cannot be neglected in comparison to $10^{-8} \,M$ The $pH$ can be calculated as follows

From acid, $\left[ H ^{+}\right]=10^{-8}\, M$

From water, $\left[ H ^{+}\right]=10^{-7}\, M$

$\therefore $ Total $\left[ H ^{+}\right] =10^{-8}+10^{-7}=10^{-8}(1+10) $

$=11 \times 10^{-8}\, M$

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

$=-\log \left(11 \times 10^{-8}\right) $

$=-\left[\log 11+\log 1{0}^{-8}\right] $

$=-[1.0414-8]=6.9586 $