Question

$H_{2}A$ is a weak diprotic acid.If the pH of 0.1 M $H_{2}A$ solution is 3 and conc. of $A^{2-}$ is ${10}^{-12}$ M at 25 :

• A. $[H^{+}{]}_{total}\simeq [H^{+}]$ from[1^{st}] step of ionization of acid $H_{2}A$
• B. conc. of $O{H}^{-}$ in solution is ${10}^{-3}M$
• C. The value of $K_{a_1}$ is nearly equal to ${10}^{-5}$
• D. $p{K}_{a_2}-p{K}_{a_1}=7$

Answer 1

The correct option is A $[H^{+}{]}_{total}\simeq [H^{+}]$ from[1^{st}] step of ionization of acid $H_{2}A$

$H_{2}A\rightleftharpoons H^{+}+H{A}^{-}$

$HA\rightleftharpoons H^{+}+A^{2-}$

$[H^{+}{]}_{total}=[H^{+}]$ from $1st$ step ionization of $H_{2}A+\left[h^{+}\right]$ from $2nd$ step ionization of $H_{2}A\simeq \left[H^{+}\right]$ from $1st$ step ionization of $H_{2}A$ .

[Since the concentration of $A^{2-}$ produced from $2nd$ step ionization is ${10}^{-12}M$ which is very negligible]