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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=5 is obtained ?

Given :Ka value of acetic acid is 1.8×105 log101.8=0.25 and 10(0.25)=1.8
  1. 1:1.8
  2. 1.8:1
  3. 0.18:1
  4. 1:0.18


Solution

The correct option is B 1.8:1
Given, Ka(acetic acid)=1.8×105 at 25oC, pKa=log10[1.8×105]pKa=5log 1.8pKa=50.25pKa=4.75 Given , pH=5
Formula of pH for an acidic buffer solution is :
pH=pKa+log[anion of salt][acid]
5=4.75+log[sodium acetate][acetic acid]
0.25=log[sodium acetate][acetic acid][sodium acetate][acetic acid]=10(0.25)
[sodium acetate][acetic acid]=1.8
So, by mixing sodium acetate and acetic acid in the ratio 1.8:1 mole per dm3.
The buffer of pH=5 is obtained. 

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