Consider an acidic buffer solution of acetic acid and sodium acetate at 25oC. What should be the ratio of concentration of sodium acetate and acetic acid so that pH=4 is obtained ?
Given :Ka value of acetic acid is 1.8×10−5and10(−0.75)=0.18
A
0.18:1
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B
1:0.18
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C
1:1.81
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D
None of the above
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Solution
The correct option is A0.18:1 Given, Ka(acetic acid)=1.8×10−5at25oC,pKa=−log10[1.8×10−5]pKa=5−log1.8pKa=5−0.25pKa=4.75Given , pH=4
Formula of pH for an acidic buffer solution is : pH=pKa+log[anion of salt][acid] ⇒4=4.75+log[sodium acetate][acetic acid] ⇒−0.75=log[sodium acetate][acetic acid][sodium acetate][acetic acid]=10(−0.75) [sodium acetate][acetic acid]=0.18
So, by mixing sodium acetate and acetic acid in the ratio 0.18:1.
The buffer of pH=4 is obtained.