Chemistry: The Molecular Science (5th Edition)

Published by Cengage Learning
ISBN 10: 1285199049
ISBN 13: 978-1-28519-904-7

Chapter 15 - Additional Aqueous Equilibria - Questions for Review and Thought - Topical Questions - Page 693: 15c

Answer

$NH_3$ and $NH_4Cl$ are the best combination for a buffer with $pH \approx 9$ because, the $pKa$ for $N{H_4}^+$ is about $9$.

Work Step by Step

1. Identify the acid: $N{H_4}^+$. 2. Calculate its $pK_a$ ** Use the Kb from its conjugate base. $K_b * K_a = K_w = 10^{-14}$ $ 1.8\times 10^{- 5} * K_a = 10^{-14}$ $K_a = \frac{10^{-14}}{ 1.8\times 10^{- 5}}$ $K_a = 5.556\times 10^{- 10}$ $pKa = -log(Ka)$ $pKa = -log( 5.556 \times 10^{- 10})$ $pKa = 9.25$ 3. Compare this value to 9. - 9.25 is close to 9, therefore, this combination is a good choice.
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