# A 0.10 M aqueous solution of sodium dihydrogen phosphate, NaH 2 PO 4 , has a pH of 4.10. Calculate K a 2 for phosphoric acid. You can ignore hydrolysis of the H 2 PO 4 − ion.

### General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
Publisher: Cengage Learning
ISBN: 9781305580343

### General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
Publisher: Cengage Learning
ISBN: 9781305580343

#### Solutions

Chapter
Section
Chapter 16, Problem 16.98QP
Textbook Problem

## A 0.10 M aqueous solution of sodium dihydrogen phosphate, NaH2PO4, has a pH of 4.10. Calculate Ka2 for phosphoric acid. You can ignore hydrolysis of the H2PO4− ion.

Expert Solution
Interpretation Introduction

Interpretation:

The Ka2 value for phosphoric acid has to be calculated

Concept Introduction:

Acid ionization constant Ka :

The ionization of a weak acid HA can be given as follows,

HA(aq)H+(aq)+A-(aq)

The equilibrium expression for the above reaction is given below.

Ka=[H+][A-][HA]

Where,

Ka is acid ionization constant,

[H+] is concentration of hydrogen ion

[A-] is concentration of acid anion

[HA] is concentration of the acid

Diprotic and polyprotic acids:

Acids having two or more hydrogen atoms are termed as diprotic or polyprotic acids.  These acids lose one proton at a time by undergoing successive ionizations.

For diprotic acids, the successive ionization constants are designated as Ka1andKa2

For triprotic acids, the successive ionization constants are designated as Ka1,Ka2andKa3

### Explanation of Solution

To Calculate: The Ka2 value for phosphoric acid

Given data:

The conc. of the aqueous solution of sodium dihydrogen phosphate (NaH2PO4) = 0.10 M

The pH of the solution is 4.10

The hydrolysis of H2PO4- ion can be ignored.

Phosphoric acid:

Phosphoric acid is a triprotic acid.

The ionizations of phosphoric acid and their respective ionization constants are as follows,

H3PO4 + H2O H2PO4- + H3O+ Ka1H2PO4- + H2O HPO42- + H3O+ Ka2HPO42- + H2O PO43- + H3O+ Ka3

The first ionization of phosphoric acid is ignored as given. (Since, hydrolysis of H2PO4- ion can be ignored)

Ka2 value calculation:

The Ka2 value for phosphoric acid is calculated from (NaH2PO4) .

Sodium dihydrogen phosphate (NaH2PO4) contains Na+ and H2PO4- ions.

The hydronium ion concentration is calculated from the pH as follows,

[H+] =10pH =104.10 =7.94×105 M

The concentration of the hydronium ion is 7.94×105 M

The initial concentration of H2PO4- ion is:

Initial conc

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