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Question

What is the pH when 25.0 mL of 0.2 M of CH3COOH has been titrated with 35.0 mL of 0.1 M NaOH ?
pKa(CH3COOH)=4.752
log 2.333=0.368

A
7
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B
3.45
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C
5.12
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D
6.52
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Solution

The correct option is C 5.12
1) Determine moles of acetic acid and NaOH before mixing:
CH3COOH:(0.2 molL1)×(0.025 L)=0.005 mol
NaOH:(0.1 molL1)×(0.035 L)=0.0035 mol

2) Determine moles of acetic acid and sodium acetate after mixing:
CH3COOH: 0.005 mol0.0035 mol=0.0015 mol
CH3COONa:0.0035 mol

3) Use the Henderson - Hasselbalch Equation:
pH=pKa+log[[conjugate base][acid]]
pH=pKa+log[[CH3COONa][CH3COOH]]
pH=4.752+log 0.00350.0600.00150.060
pH=4.752+log 2.333
pH=4.752+0.368=5.120

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