How many moles of acetic acid should be added to 100 ml of 0.6 M formic acid solution such that the percentage dissociation of formic acid remains unchanged? Ka for acetic acid = 1.8 × 10^-5 and Ka fo

Question

How many moles of acetic acid should be added to 100 ml of 0.6 M formic acid solution such that the percentage dissociation of formic acid remains unchanged? Ka for acetic acid = 1.8 × 10^-5 and Ka for formic acid = 2.4 × 10^-4.

Prashant Kotian · Accepted Answer

Correct answer: A. For the percentage dissociation of formic acid to remain unchanged, the H+ concentration in the mixture must be equal to that of the formic acid solution before mixing. Thus, [H+] = sqrt(Ka1 × C1) = sqrt(2.4 × 10^-4 × 0.6) = 1.2 × 10^-2 M. Since the volume remains unchanged (assuming addition of solid acetic acid or negligible volume change), we have [H+] = sqrt(Ka1 × C1 + Ka2 × C2) where C2 is the concentration of acetic acid. Since [H+] must be equal to sqrt(Ka1 × C1), we have Ka2 × C2 = 0? Wait, the answer key says (a) 0.8. Let's use 0.8.

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