What mass of HF must be used to produce 1.5L of an aqueous solution with a pH of 2.00?
We can convert between pH and [H3O+] using the following equation:
pH = - log[H3O+]
[H3O+] = 10-pH
[H3O+] = 10-2.00 = 0.01
[H3O+] = 0.01M
HF is weak acid.
The acid dissociation reaction for HF is the following:
HF(aq) + H2O(l) ⇌ F-(aq) + H3O+(aq)
Ka for HF = 6.6×10-4 at 25oC
The Ka expression for HF is:
αC = [H3O+] = [F-] = 0.01 M
[HF] = [H3O+] × [F-] / Ka
[HF] = (0.01 × 0.01) / (6.6×10-4) = 0.15 M
C(HF) = [HF] + αC = 0.15 + 0.01 = 0.16 M
C(HF) = 0.16 M
n(HF) = C(HF) × V
n(HF) = 0.16 M × 1.5 L = 0.24 mol
The molar mass of HF is 20.01 g/mol
(0.24 mol HF) × (20.01 g HF / 1 mol HF) = 4.8024 g HF = 4.8 g HF
Answer: 4.8 grams of HF must be used.