Ionic EquilibriumHard

Question

A definite volume of a N/20 CH3COOH (pKa = 4.7447) is titrated with a strong base (NaOH). It is found that 80 equal sized drops of NaOH, added from a burette effect the complete neutralisation. Find the pH, when the acid solution is neutralised to the extent of 20%.

Options

A.4.14
B.9.86
C.5.34
D.8.68

Solution

WA(CH3COOH) is titrated with SB(NaOH) and 20% of the WA is neutralised, so 20% of the salt of WA/SB (CH3COOH) is formed and 80% of the WA(CH3COOH) is left. So an acidic buffer is formed.
∴ [Acid] = 80;    [Salt] = 20
pH   = pKa + log
= 4.7447 + log
= 4.7447 + log
= 4.7447 - 2 log 2 = 4.7447 - 2 × 0.3010 = 4.14

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