   Chapter 16, Problem 25PS

Chapter
Section
Textbook Problem

If each of the salts listed here were dissolved in water to give 0.10M solution, which solution would have the highest pH? Which would have the lowest pH?(a) Na2S(b) Na3PO4(c) NaH2PO4(d) NaF(e) NaCH3CO2(f) AlCl3

Interpretation Introduction

Interpretation:

The highest pH and lowest pH has to be determined when the given solutions are dissolved in the water to give 0.10M solution.

Concept introduction:

pH is the unit of measurment for determining the acidity or alkalinity of a solution.

The mathematical definition of pH is the negative logarithm of the molar hydrogen ion concentration.

pH =  -log[H+]

Explanation

Given:

1. a) Na2S

Na2S dissociates into S2- and Na+ ions. The S2- ion abstracts a proton from water to form hydroxide ions. Therefore, the solution is basic and the pH value is greater than the 7. Na+ does not affect the pH of the solution.

S2-(aq) + H2O(l)HS- (aq) + OH-(aq)   Kb=1×105

1. b) Na3PO4

Na3PO4 dissociates into PO43- and Na+ ions. The PO43- ions abstract a proton from water to form hydroxide ions. Therefore, the solution is basic and the pH value is greater than the 7. Na+ does not affect the pH of the solution.

PO43-(aq) + H2O(l) HPO42-(aq) + OH-(aq)  Kb= 2.8×10-2

2. c) NaH2PO4

NaH2PO4 dissociates into H2PO4- and Na+ ions. The H2PO4- ion abstracts a proton from water to form hydroxide ions. Therefore, the solution is basic and the pH value is greater than the 7. Na+ does not affect the pH of the solution.

H2PO4-(aq) + H2O(l)H3PO4(aq) + OH-   Kb=1

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