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Question

Calculate the pH of a buffer solution made from 0.20 M HC2H3O2 and 0.50 M C2H3O2 that has an acid dissociation constant for HC2H3O2 of 1.8×105. 
log(1.8 )=0.255, log(2.5 )=0.397


A
7
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B
5.14
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C
5.64
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D
4.35
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Solution

The correct option is B 5.14
Given, [C2H3O2]=0.50 M, [HC2H3O2]=0.2 M, Ka=1.8×105.  
Using the Henderson-Hasselbalch equation for weak acid:
pH=pKa+log[[conjugate base][acid]]
pH=pKa+log ([A][HA])
pH=pKa+log([C2H3O2][HC2H3O2])
pH=log(1.8×105)+log(0.500.20)
pH=log(1.8×105)+log(2.5)
pH=5log(1.8)+log(2.5)
pH=50.255+0.397
pH=5.14

Chemistry

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