Chapter 17.6, Problem 2.1ACP

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# Phosphate ions are abundant in cells, both as the ions themselves and as important substituents on organic molecules. Most importantly, the pKa for the H2PO4− ion is 7.20, which is very close to the normal pH in the body.H2PO4−(aq) + H2O(ℓ) ⇄ H3O+(aq) + HPO42−(aq)1. What should the ratio [HPO42−]/[H2PO4−] be to control the pH at 7.40?

Interpretation Introduction

Interpretation:

The ratio [HPO42]/[H2PO4] so that the pH is controlled at 7.40 has to be calculated.

Concept introduction:

The pH value of the body is maintanined in the range of 6.97.4 by buffer system. The two important buffer system present in the biological system are bicarbonate/carbonic acid system and phosphate system.

The pH is related to the concentration of conjugate base and acid as follows,

pH=pKa+log([conjugate base][weak acid])

This equation is known as Handerson-Hasselbalch equation.

Explanation

The ratio [HPO42âˆ’]/[H2PO4âˆ’] so that the pH is controlled at 7.40 is calculated below.

Given:

The pKa value for weak acid H2PO4âˆ’ is 7.2.

The phosphate equilibria is given as,

â€‚Â H2PO4âˆ’(aq)+H2O(aq)â†’H3O+(aq)+HPO42âˆ’(aq)

HPO42âˆ’ is the conjugate base of weak acid H2PO4âˆ’.

The pH is related to the concentration of conjugate base, HPO42âˆ’ and acid, H2PO4âˆ’ as follows,

pH=pKa+log([HPO42âˆ’][H2

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