Question

A buffer system is prepared by mixing 2 mols of HF(aq) (Ka=7.1 x 10^-4) and 1.8 moles NaF(aq) in a 500 mL solution. A) calculate the pH of the buffer. B) calculate the pH of the buffer after adding 0.2 mols of HCl. C) calculate the pH of the buffer after adding 0.3 mols LiOH

Answer 1

C_M (HF) = 2mol / 0,5 liters = 4 mol/l

C_M (NaF) = 1,8mol / 0,5 liters = 3,6 mol/l

K_a = 7,1*10^-4

A.)

pK_a = -log (K_a)

pK_a = -log(7,1*10^-4) = 3,15

Henderson-Hasselbalch equation:

$pH= pK_a + \cfrac {[A^-]}{[HA]}$

$pH = 3,15 + \cfrac {3,6}{4}= \bold{4,05}$

B.)

C_M(HCl) = 0,2 mol / 0,5 litres = 0,4 mol/l

NaF + HCl = NaCl + HF

3,6-0,4 – 0,4-0,4 – 0,4 – 4 + 0,4

[A^–] = 3,2 mol/l

[HA] = 4,4 mol/l

$pH= 3,15+ \cfrac {3,2}{4,4}= \bold{3,88}$

C.)

C_M(LiOH) = 0,3mol / 0,5 litres = 0,6 mol/l

HF + LiOH = LiF + H₂O

4-0,6 – 0,6-0,6 – 0,6 – 0,6

[A^–] = 3,6 + 0,6 = 4,2

[HA] = 4 - 0,6 = 3,4

$pH= 3,15+ \cfrac {4,2}{3,4}= \bold{4,385}$