Answer to Question #196774 in Chemistry for hillary

Question

What mass of HF must be used to produce 1.5L of an aqueous solution with a pH of 2.00? 

Answer 1

Solution:

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)

K_a for HF = 6.6×10^-4 at 25^oC

The K_a expression for HF is:

ICE Table: 

αC = [H₃O⁺] = [F^-] = 0.01 M

Hence,

[HF] = [H₃O⁺] × [F^-] / K_a

[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

Hence,

(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.