Question

The $pH$ of an aqueous solution of a $1\times {10}^{-7}M$ solution of $HCl$ will be

• A. 7
• B. slightly less than 7
• C. slightly greater than 7
• D. 1

Answer 1

Answer: (B)

$[H^{+}]$ ions from $HCl$ acid $=1\times {10}^{-7}M$ Here, $[H^{+}]$ ions from $H_{2}O={10}^{-7}M$ cannot be neglected in comparison to ${10}^{-7}M[H^{+}]$ ions from $HCl$ . $\therefore$ Total $[H^{+}]={10}^{-7}+{10}^{-7}$ $={10}^{-7}(1+1)$ $=2\times {10}^{-7}M$ $pH=-\log [H^{+}]$ $=-\log [2\times {10}^{-7}]$ $=-[\log 2+\log {10}^{-7}]$ $=-(0.3010-7)$ $=6.6989$ $\therefore$ $pH$ of a solution of $1\times {10}^{-7}M$ is slightly less than 7.