1. What change will be caused by the addition of a small amount of HCl to a solu

Question

1. What change will be caused by the addition of a small amount of HCl to a solution containing roughly equimolar amounts of HF and NaF?

a) conc of hydronium ions would increase significantly

b) conc of flouride ions and conc of hydronium ions will both increase

c) conc of hydrogen flouride will decrease and conc of flouride will increase

d) conc of flouride ions will decrease and conc of hydrogen flouride will increase

e) the flouride ions will precipitate from solution

I know the answer is d, I just don't understand why. If you could please explain!

2. Of the following, which soln has the greatest buffering capacity?

a) .821 M HF and .217 M NaF

b) .821 M HF and .909 M NaF

c) .100 M HF and .217 M NaF

d) .121 M HF and .667 M NaF

please explain why!

Explanation / Answer

1. What change will be caused by the addition of a small amount of HCl to a solution containing roughly equimolar amounts of HF and NaF?

HF <--> H+ and F-

NaF --> Na+ and F-; there is common ion effect, F- from Na+ will not form NaF, but F- from HF, may form HF again

Adding H+ will increase the concnetration of products, therefore force more HA creation (lechatelier principle) the shift goes to the left, more HF is formed

a) conc of hydronium ions would increase significantly

b) conc of flouride ions and conc of hydronium ions will both increase

c) conc of hydrogen flouride will decrease and conc of flouride will increase

d) conc of flouride ions will decrease and conc of hydrogen flouride will increase

e) the flouride ions will precipitate from solution

2. Of the following, which soln has the greatest buffering capacity?

Greates buffer capacity is either acidic and basic, therefore you must have the pH equation

pH = pKA + log(A-/HA)

since you want to have both, acidic and basic capacities, you must have the nearest value of log(A-/HA)

that is log(1) or a pH similar to that of the pKa and the vlaues of F- = HF must be pretty similar

the neares answer will be the raito = 1

a) .821 M HF and .217 M NaF = 3.78

b) .821 M HF and .909 M NaF = 1.107

c) .100 M HF and .217 M NaF = 2.17

d) .121 M HF and .667 M NaF = 5.51

Best option is b, 1.107 ratio has the best buffer capacity

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