Question

Find the $\Delta pH$ (initial pH-final pH) when 100 ml 0.01 M HCl is added in a solution containing 0.1 mili moles of $NaHC{O}_3$ solution of negligible volume. (inital $pH=9,K{a}_1={10}^{-7},K{a}_2={10}^{-11}$ for $H_{2}C{O}_3$).

• A. 6 + 2 log 3
• B. 6 $-$ log 3
• C. 6 + 2 log 2
• D. 6 $-$ 2 log 3

Answer 1

The correct option is A 6 + 2 log 3

The initial pH is the pH of the $NaHC{O}_3$ solution. It is 9.

The reaction between HCl and $NaHC{O}_3$ can be represented as, $H^{+}+HC{O}_3^{+}\to H_{2}C{O}_3$.

The number of mmoles of HCl remaining unreacted is $=1-0.1=0.9mmol$.

The expression for the final pH of the solution is $pH=-log(9\times {10}^{-3})=-2log3+3$.

The change in pH is $(initialpH-finalpH)=9-(-2log3+3)=6+2log3$.