Question

What is the $pH$ of $0.01M$ glycine solution?
For glycine $K_{a_1}=4.5\times {10}^{-3}$ and $K_{a_2}=1.7\times {10}^{-10}$ at $298K$

• A. $3.0$
• B. $10.0$
• C. $7.06$
• D. $8.2$

Answer 1

The correct option is C $7.06$

Glycine $H_3{N}^{+}{CH}_{2}CO{O}^{-}$ is more acidic than basic. Instead of $K_b$ value the second given $K_a$ value is corresponding to the $K_b$ value ($\because K_a\times K_b={10}^{-14}$) Hence, overall ionisation constant,

$K=K_{a_1}\times K_{a_2}=4.5\times {10}^{-3}\times 1.7\times {10}^{-10}=7.65\times {!0}^{-13}$

$H^{+}=\sqrt{KC}=\sqrt{7.65\times {10}^{-13}\times 0.01}$=$0.87\times {10}^{-7}M$

$pH=-\log (0.87\times {10}^{-7})=7.06$