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Question

20 mL of 0.2 M CH3COONa solution is titrated against 0.1 M HCl. Calculate the pH of solution when 10 mL HCl has been added.
Given: pKa(CH3COOH)=4.74

A
5.22
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B
6.44
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C
3.55
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D
7.64
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Solution

The correct option is A 5.22
mmol of CH3COONa=20×0.2=4
mmol of HCl=10×0.1=1

CH3COONa (aq)+HCl (aq)CH3COOH (aq)+NaCl (l)Initial: 4 1 0 0Final: 3.0 0 1 1

After titration we can see that 3.0 mmol of CH3COONa and 1 mmol of CH3COOH are present. So , the resultant solution will act as an buffer.
Final concentration=Moles Total Volume
Total Volume =20+10=30 mL
[CH3COOH]final=130 M
[CH3CONa]final=330 M
The formula to calculate the pH of an acidic buffer is :
pH=pKa+log([CH3COONa][CH3COOH])pH=4.74+log330130pH=4.74+log(3)=4.74+0.48pH=5.22

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