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Chapter 17.6, Problem 2.2ACP
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### Chemistry & Chemical Reactivity

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

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Section
BuyFindarrow_forward

### Chemistry & Chemical Reactivity

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

# A typical total phosphate concentration in a cell, [HPO42−] + [H2PO4−], is 2.0 × 10−2 M. What are the concentrations of HPO42− and HPO4− at pH 7.40?

Interpretation Introduction

Interpretation:

The concentration of [H2PO4] and [HPO42] at pH7.40 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 concentration ofÂ  [H2PO4âˆ’] and [HPO42âˆ’] at pH7.40 is calculated below.

Given:

The total phosphate concentration ([H2PO4âˆ’]+[HPO42âˆ’]) is 2.0Ã—10âˆ’2Â M.

The pKa value for weak acid H2PO4âˆ’ is 7.2.

The pH of system is 7.40.

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’][H2PO4âˆ’])

Rearrange for [HPO42âˆ’]/[H2PO4âˆ’].

[HPO42âˆ’][H2PO4âˆ’]=10(pHâˆ’pKa)

Substitute the values to calculate the value of ratio of [HPO42âˆ’]/[H2PO4âˆ’].

[HPO42âˆ’][H2PO4âˆ’]=10(7

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