Chemistry: The Molecular Science (5th Edition)

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

Chapter 14 - Acids and Bases - Questions for Review and Thought - Topical Questions - Page 652d: 71a


The pH of an $Na_2HPO_4$ solution will be greater than 7. This happens because, this salt is formed by a insignificant acid $(Na^+)$ and a weak base** $(HP{O_4}^-)$, therefore, the solution is basic. ** This compound is amphiprotic, but, since its $K_b$ value is larger than its $K_a$ value, it will act more like a base.

Work Step by Step

To predict the pH range of an salt: 1. Identify the ions that form this salt, and compare its strengths: 2 $Na^+$: Conjugate acid of a strong base $(NaOH)$: Insignificant acid. 1 ${HPO_4}^{2-}$: Amphiprotic compound, but it is more basic than it is acidic $(K_b > K_a)$ Therefore: ${HPO_4}^{2-}$ is the stronger electrolyte, so, the solution is basic. ** You can find the constants values on page 624, table 14.2.
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