A chemist prepares a 0.100 M aqueous solution of NH4F. A table of ionization con

ID: 508460 • Letter: A

Question

A chemist prepares a 0.100 M aqueous solution of NH4F. A table of ionization constants gives the following at 25 °C:

NH3 + H2O(l) = NH4 + + OH pKb = 4.74

HF + H2O(l) = H3O + + F pKa = 3.18

a) Determine the pKa of NH4 + and the pKb of F- at 25 °C.

b) Will the solution be acidic, basic, or neutral? Briefly explain your answer.

c) A strong acid is added to this solution until the pH is 3.50. Calculate the ratio of [F- ] to [HF] in the resulting solution.

d) Once the pH is adjusted to 3.50, will the pH be sensitive to the addition of a few more drops of strong acid? Give a reason for your answer.

Recall that

pKa + pKb = 14

determine pKa NH4:

pKa = 14-pKb = 14-4.74 = 9.26

determine pKb F-:

pKb = 14-pKa = 14-3.18 = 10.82

First, identify:

There will be higher H3O+ since Ka is higher for HF than Kb for OH-

H3O > OH- this will be acidic

c) A strong acid is added to this solution until the pH is 3.50. Calculate the ratio of [F- ] to [HF] in the resulting solution.

pH = 4.50

find F-/HF ratio

pH = pKa + log(F-/HF)

3.50 = 3.18 + log(ratio)

ratio = 10^(3.50-3.18) = 2.08929

d) Once the pH is adjusted to 3.50, will the pH be sensitive to the addition of a few more drops of strong acid? Give a reason for your answer.

No... it will not be sensitive, since a buffer is formed, so there are equilibriums shifting the pH ranges

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