Practice Exercise 16.17 When butter turns rancid, its foul odor is mostly that o

Question

Practice Exercise 16.17 When butter turns rancid, its foul odor is mostly that of butanoic acid, HC4C,02, a weak monoprotic acid similar to acetic acid in structure. In a 0.0100 M solution of butanoic acid at 20 °C, the acid is 4.0% ionized. Calculate the Ka of butanoic acid at this temperature. The number of significant digits is set to 2; the tolerance is 1 in the 2nd significant digit Calculate the pK, of butanoic acid at this temperature The number of significant digits is set to 3; the tolerance is +-1 in the 3rd significant digit

Explanation / Answer

Given that

dissociation constant, a = 0.04 (or 4%)

At equilibrium

HA in equilibrium H+ + A-

C(1-a) Ca Ca

Ka = [H+] [A-]/[HA] = C2a2/C(1-a) (since a is very small on comparing with 1)

Ka = Ca2

Ka = 0.01 x 0.04x0.04 = 0.000016

pKa = -log Ka = -log 0.000016= 4.795

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