## Chemistry and Chemical Reactivity (9th Edition)

Published by Cengage Learning

# Chapter 17 Principles of Chemical Reactivity: Other Aspects of Aqueous Equilibria - Study Questions - Page 677a: 18

#### Answer

(b) $NaH_2PO_4$ and $Na_2HPO_4$ are the best choice for this situation.

#### Work Step by Step

1. First, determine the $pK_a$ for the acid in each pair: (a) 1. Calculate the pKa value $(H_3P{O_4}):$ $pKa = -log(Ka)$ $pKa = -log( 7.5 \times 10^{- 3})$ $pKa = 2.12$ (b) 1. Calculate the pKa value $H_2P{O_4}^-:$ $pKa = -log(Ka)$ $pKa = -log( 6.2 \times 10^{- 8})$ $pKa = 7.21$ (c) 1. Calculate the pKa value $(H{PO_4}^{2-})$: $pKa = -log(Ka)$ $pKa = -log( 3.6 \times 10^{- 13})$ $pKa = 12.44$ 2. Identify the acid with the closest $pK_a$ to the desired pH (7): - It is the $H_2P{O_4}^-$. Therefore: Correct answer = (b)

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