   Chapter 20, Problem 50E

Chapter
Section
Textbook Problem

# Phosphate buffers are important in regulating the pH of intracellular fluids. If the concentration ratio of H2PO4−/HPO42− in a sample of intracellular fluid is 1.1: 1, what is the pH of this sample of intracellular fluid? H 2 PO 4 − ( a q ) ⇌ HPO 4 2 − ( a q ) + H + ( a q )           K a = 6.2 × 10 − 8

Interpretation Introduction

Interpretation: The concentration ratio of H2PO4/HPO42 and the value of Ka is given for the reaction of phosphate buffer solution. The pH value is to be calculated.

Concept introduction: The pH value is the measure of H+ ions. The relation between pH and pKa is given by Henderson-Hasselbach equation. According to this equation,

pH=pKa+log[Conjugatebase][Weakacid]

To determine: The pH value of a sample of intracellular fluid.

Explanation

Explanation

Given

The value of Ka for the given reaction is 6.2×108 .

The concentration ratio of H2PO4/HPO42 is,

H2PO4HPO42=1.11

The stated reaction is,

H2PO4(aq)HPO42(aq)+H+(aq)

Formula

The formula of pKa is,

pKa=logKa

Where,

• Ka is acid equilibrium constant.
• pKa is the measure of acidic strength.

Substitute the value of Ka in the above equation.

pKa=logKa=log(6.2×108)=7.21_

The pH value of solution is 7.17_ .

The concentration ratio of H2PO4/HPO42 is,

H2PO4HPO42=1

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