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Question

A 30 mL solution of 0.2 M CH3COOH is titrated with 0.2 M NaOH. Find the pH of the solution when 10 mL of NaOH has been added.
Given pKa(CH3COOH)=4.74

A
5.04
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B
4.44
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C
3.54
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D
4.04
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Solution

The correct option is B 4.44
mmol of CH3COOH=30×0.2=6
mmol of NaOH=10×0.2=2

CH3COOH (aq)+NaOH (aq)CH3COONa (aq)+H2O (l)Initially: 6 2 0 0Final: 4 0 2 2

After titration we can see that 4 mmol of CH3COOH and 2 mmol of CH3COONa are present.
Final Concentration =Moles Total Volume
Total Volume =30+10=40 mL
[CH3COOH]=440 M
[CH3COONa]=240 M
An acidic buffer is formed. So,
pH=pKa+log([CH3COONa][CH3COOH])pH=4.74+log240440pH=4.74+log(12)=4.740.3pH=4.44

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