[bookworm]

Question

Hydrofluoric acid, HF, is a weak acid with an acid dissociation constant, Ka, of [6.6 * 10^-4]

a) Explain whether a 0.1M acetic acid or 0.1M hydrofluoric acid will have a lower pH value.
(Ka of acetic acid = 1.8*10^-5)

my explanation:
As Ka is bigger, the stronger the acid. Since Ka of HF is higher, therefore the pH value of HF will be lower.

b)should a solution of NaF be acidic, basic or neutral? Explain your answer.

my explanation:

NaF  Na+(do not undergo salt hydrolysis)  +  F-
F- + H2O  HF + OH-
.:. the solution of NaF is basic

c) If a 325ml solution contains 0.8M HF, what is the mass of NaF required to prepare a buffer soltuin with pH 3.5?

my answer:

pH = pKa + log (salt/acid)
3.5 = -lg(6.6*10^-4) + lg (salt/0.
molarity of salt (NaF) = 1.67M
No. of moles of NaF = 1.67M * 0.325L = 0.543moles
Mr of NaF = 19+23 = 42
Mass of NaF = 42*0.543 = 22.8g

d)with the help of an eqn, explain what happens when a small amount of NaOH is added to the solution in (c).

NaOH + HF  NaF + H2O       
NaF  Na+  + F-
F- + H2O  HF + OH-

[Borek]

First two are OK, as for the third, you seem to forgot that NaOH and NaF are fully dissociated.

Besides, I suppose they want you to explain what happens to the pH.

[bookworm]

the pH will be higher?

[Borek]

Well, it is rather unlikely that pH goes down when you add a strong base.

However, pH changes much less than one may expect.

[bookworm]

o.o..
i think i somehow get it.. thanks