   Chapter 17.6, Problem 2.1ACP

Chapter
Section
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 [HPO42]/[H2PO4] so that the pH is controlled at 7.40 is calculated below.

Given:

The pKa value for weak acid H2PO4 is 7.2.

The phosphate equilibria is given as,

H2PO4(aq)+H2O(aq)H3O+(aq)+HPO42(aq)

HPO42 is the conjugate base of weak acid H2PO4.

The pH is related to the concentration of conjugate base, HPO42 and acid, H2PO4 as follows,

pH=pKa+log([HPO42][H2

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