9. Which of the following solutions has the greatest buffering
capacity? [A] 0.243 M HF and 0.192 M NaF [B] 0.485 M HF and 0.383 M NaF [C] 0.970 M HF and 0.766 M NaF [D] 0.160 M HF and 0.126 M NaF [E] They are all buffer solutions and would all
have the same capacity. 10. The Henderson-Hasselbalch equation is
________ [A] pH = pKa + log
[acid]/[base] a acid base [B] [H+] = Ka +
[base]/[acid] + a base acid [C] pH = log [acid]/[base] acid base [D] pH = pKa – log
[base]/[acid] a base acid [E] pH = pKa + log
[base]/[acid] a base acid 7. Calculate the pH of a solution prepared by dissolving 0.150
mol of benzoic acid and 0.300 mol of sodium benzoate in water sufficient to yield
1.00 L of solution. The Ka of benzoic a acid is 6.30 x 10-5. -5 [A] 2.516 [B] 4.502 [C] 3.892 [D] 4.195 [E] 10.158 8. What change will be caused by addition of a small amount of
HCl to a solution containing fluoride ions and hydrogen fluoride? [A] The fluoride ions will precipitate out of solution as its
acid salt. [B] The concentration of fluoride ion will decrease and the
concentration of hydrogen fluoride will increase. [C] The concentration of hydronium ions will increase
significantly [D] The concentration of hydrogen fluoride will decrease and the
concentration of fluoride ions will increase. [E] The concentration of fluoride ions will increase as will the
concentration of hydronium ions. 11. The addition of hydrofluoric acid and ________ to water
produces a buffer solution [A] HF [B] KI [C] NaF [D] NaBr [E] NaNO3
3
Buffer solution is a solution which resists the change in
pH.
Buffer solutions are of two types.
1. Acidic buffer: It is a mixture of weak acid and salt formed
by this weak acid with a strong base.
Ex: Acetic acid+ Sodium acetate
2. Basic buffer: It is a mixture of weak base and salt formed by
this weak base with a strong acid.
Ex: Ammonium hydroxide (weak base) and Ammonium chloride (salt
formed by Ammonium hydroxide with HCl)
pH of acidic buffer is given by
pH = pKa + log([Salt]/[Acid]) or
pH = pKa + log([Conjugate base]/[Acid])
pOH of basic buffer is given by
pOH = pKb + log([Salt]/[Base])
1)
For optimal buffer, the ratio of [Conjugate base]/[Acid] should
be 1.
The given buffer is HF (Hydrofluoric acid) and NaF (Sodium
fluoride).
Option A : [NaF]/[HF] = 0.192 M/0.243 M = 0.790
Option B: [NaF]/[HF] = 0.383 M/0.485 M = 0.790
Option C: [NaF]/[HF] = 0.766 M/0.970 M = 0.790
Option D: [NaF]/[HF] = 0.126 M/0.160 M = 0.788
All are having same buffer capacity.
Option E is the correct answer
10)
pH of acidic buffer is given by
pH = pKa + log([Salt]/[Acid]) or
pH = pKa + log([Conjugate base]/[Acid])
pH = pKa + log([Base]/[Acid])
7)
Given that Ka of benzoic acid = 6.30 x 10-5.
pKa = -log(Ka) = -log(6.30 x 10-5) = 4.20
pH = 4.201 + log(0.300/0.150)
pH = 4.201 +0.3010 = 4.502
Option B is the correct answer
8)
When HCl is added to HF and NaF buffer,
The Hydronium ions from HCl combines with Fluoride ions in the
above equilibrium and forms as HF.
Hence concentration of HF increases and concentration of
Fluoride ions (NaF) decreases.
Option B is the correct answer
11)
According to the definition of buffer solution is a solution
which resists the change in pH
We know that Acidic buffer: It is a mixture of weak acid and
salt formed by this weak acid with a strong base.
HF is a weak acid. Among the given options, NaF is the salt of
HF (weak acid) with NaOH(strong base).
The addition of hydrofluoric acid and NaF to water produces a
buffer solution
Option C is the correct answer.
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