Calculate the pH of a buffer solution made from 0.20 MHC2H3O2 and 0.50 M C2H3O−2 that has an acid dissociation constant for HC2H3O2 of 1.8×10−5.
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, [C2H3O−2]=0.50M,[HC2H3O2]=0.2M,Ka=1.8×10−5.
Using the Henderson-Hasselbalch equation for weak acid: pH=pKa+log[[conjugate base][acid]] pH=pKa+log([A−][HA]) pH=pKa+log([C2H3O−2][HC2H3O2]) pH=−log(1.8×10−5)+log(0.500.20) pH=−log(1.8×10−5)+log(2.5) pH=5−log(1.8)+log(2.5) pH=5−0.255+0.397 pH=5.14