JEE PYQ: Ionic Equilibrium Question 9
Question 9 - 2021 (26 Feb Shift 2)
Type: Numerical
The pH of ammonium phosphate solution, if $\text{pK}_a$ of phosphoric acid and $\text{pK}_b$ of ammonium hydroxide are 5.23 and 4.75 respectively, is
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Answer: 7
Solution
$(\text{NH}_4)_3\text{PO}_4 \rightarrow 3\text{NH}_4^+ + \text{PO}_4^{3-}$
$\text{pH} = \text{pK}_a + \frac{1}{2}(\text{pK}_w - \text{pK}_a - \text{pK}_b)$
$= 5.23 + \frac{1}{2}(14 - 5.23 - 4.75)$
$= 5.23 + \frac{1}{2}(4.02) = 7.24 \approx 7$
BETA
