Question

In $H_{3}P{O}_4$ which of the following is true?

• A. $K_a=K_{a_1}\times K_{a_2}\times K_{a_3}$
• B. $K_{a_1}<K_{a_2}<K_{a_3}$
• C. $K_{a_1}>K_{a_2}>K_{a_3}$
• D. $K_{a_1}=K_{a_2}=K_{a_3}$

Answer 1

Answer: (A), (C)

$K_a=K_{a_1}\times K_{a_2}\times K_{a_3}$
$K_{a_1}>K_{a_2}>K_{a_3}$

$H_{3}P{O}_4+H_{2}O\rightleftharpoons H_{2}P{o}_a^{-}+H_3{O}^{+}$ $K_{a_1}$= $\frac{\left[H_{2}P{O}_4^{-}\right]\left[H_3{O}^{+}\right]}{\left[H_{3}P{O}_4\right]}=7.5\times {10}^{-3}$

$H_{2}P{O}_4^{-}+H_{2}O\rightleftharpoons HP{O}_4^{-}+H_3{O}^{+}$ $K_{a_2}$=$\frac{\left[HP{O}_4^{2-}\right]\left[\right[H_3{O}^{+}]}{\left[H_{2}P{O}_4^{-}\right]}=6.23\times {10}^{-8}$

$HP{O}_4^{-2}+H_{2}O\rightleftharpoons P{O}_4^{-3}+H_3{O}^{+}$ $K_{a_3}$ =$\frac{\left[P{O}_4^{3-}\right]\left[H_3{O}^{+}\right]}{\left[HP{O}_4^{2-}\right]}=2.2\times {10}^{-13}$

From the figure it can be seen that,

$K_a=K_{a_1}\times K_{a_2}\times K_{a_3}$

and

$K_{a_1}>K_{a_2}>K_{a_3}$