Calculate the pH of an aqueous solution of $1.0 M$ ammonium formate assuming complete dissociation. $p K_{a}$ for formic acid $= 3.8$ and $p K_{b}$ of ammonia $= 4.8$

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Solution

Ammonium formate is a salt of weak acid and weak base. Hence, we apply the formula,
$p K_{a} = 3.8,$ $p K_{b} = 4.8$
$p H = 7 + \frac{1}{2} p K_{a} - \frac{1}{2} p K_{b}$
$= 7 + (1 / 2) \times 3.8 - (1 / 2) \times 4.8 = 7 + 1.9 - 2.4 = 6.5$

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