Question

Orthophosphoric acid $\left(H_3{PO}_4\right)$ is a weak tribasic mineral acid,
$H_3{PO}_4\rightleftharpoons H^{+}+H_2{PO}_4^{-};K_{a_1}={10}^{-3}$
$H_2{PO}_4^{-}\rightleftharpoons H^{+}+H{PO}_4^{-2};K_{a_2}={10}^{-7}$
$HP{O}_4^{-2}\rightleftharpoons H^{+}+{PO}_4^{-3};K_{a_3}={10}^{-13}$
In a solution of $0.1M$ $H_3{PO}_4$, concentration of ${PO}_4^{3-}$ is:

• A. ${10}^{-20}$
• B. ${10}^{-23}$
• C. ${10}^{-15}$
• D. ${10}^{-12}$

Answer 1

Answer: (B)

${10}^{-23}$

$H_{3}P{O}_4\rightleftharpoons H^{+}+H_{2}P{O}_4^{-};K_{a1}={10}^{-3}$

$H_{2}P{O}_4^{1-}\rightleftharpoons H^{+}+HP{O}_4^{2-};K_{a2}={10}^{-7}$

$HP{O}_4^{2-}\leftrightharpoons H^{+}+P{O}_4^{3-};K_{a3}={10}^{-13}$

$H_{3}P{O}_4\leftrightharpoons 3H^{+}+P{O}_4^{3-};K_0={10}^{-23}$