Calculate the exact pH of an aqueous solution of ammonium formate () at , assuming complete dissolut — Ionic Equilibrium Chemistry Question
Question
Calculate the exact pH of an aqueous solution of $1.0 \text{ M }$ ammonium formate ($HCOONH_4$) at $25^\circ C$, assuming complete dissolution. Given that the dissociation constant $pK_a$ for formic acid is exactly $3.8$ and the $pK_b$ of ammonia is exactly $4.8$.
Answer: 6.5
💡 Solution & Explanation
Ammonium formate is a salt derived from a weak acid ($HCOOH$) and a weak base ($NH_3$). The pH of its aqueous solution is given by the formula $pH = 7 + \frac{1}{2}(pK_a - pK_b)$. The concentration term does not appear in this unapproximated formula. Substituting the known values: $pH = 7 + \frac{1}{2}(3.8 - 4.8) = 7 + \frac{1}{2}(-1.0) = 7 - 0.5 = 6.5$.
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