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 693a: 18b


$H_2P{O_4}^-$ and $HP{O_4}^{2-}$ are capable of making a buffer with $5.2 \times 10^{-8}M$ of $H_3O^+$. This happens because the $K_a$ value for $H_2P{O_4}^-$ is very close to this hydronium ion concentration value. ** You can also use $HClO$ and $ClO^-$, but $H_2P{O_4}^-$ and $HP{O_4}^{2-}$ are more suitable.

Work Step by Step

1. Analyze the $[H_3O^+] (5.2 \times 10^{-8}M)$, and find the conjugate acid-base pair with the closest $K_a$ value. - Use Table $15-1$ on page $659$. - $H_2P{O_4}^-$ and $HP{O_4}^{2-}$: $K_a$ = $6.3 \times 10^{-8}$ Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $5.2 \times 10^{-8}$ ** $HClO$ and $ClO^-$ are capable of making this buffer too, but $H_2P{O_4}^-$ and $HP{O_4}^{2-}$ have a closer $K_a$ value.
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