Question

The $pH$ of ${10}^{-6}$M $HCl$, ${10}^{-7}$M $HCl$ and ${10}^{-8}$ M $HCl$ respectively is:

• A. $6,7,8$
• B. $6,6.79,6.98$
• C. $6,6.79,7.02$
• D. $6,6.98,8$

Answer 1

The correct option is B $6,6.79,6.98$

• We know $pH=-log\left(\right[H^{+}\left]\right)$
  

So,when $\left[H^{+}\right]={10}^{-6}M$, we may neglect concentration of water.
$pH=-log\left({10}^{-6}\right)=6$

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

Here we have to consider concentration of water i.e. ${10}^{-7}M$

So total concentration of $H^{+}=2\times {10}^{-7}M$

Hence, $pH=-log(2\times {10}^{-7})=6.79$

• Now, $\left[H^{+}\right]={10}^{-7}M$
Here we have to consider concentration of water i.e. ${10}^{-7}M$
So total concentration of $H^{+}={10}^{-8}+{10}^{-7}M=1.1\times {10}^{-7}M$

Hence, $pH=-log(1.1\times {10}^{-7})=6.98$