Hydrofluoric acid is a weak acid with a K_a = 6.8 times 10^-4. A titration of 25

Question

Hydrofluoric acid is a weak acid with a K_a = 6.8 times 10^-4. A titration of 25.0 mL of 0.500 M hydrofluoric acid is Carried out using 0.200 M NaOH. What is the initial pH of the hydrofluoric acid solution? Write the chemical reaction equation for this titration. 50.0 mL of 0.200 M NaOH is added to the initial HF solution. At what point in the titration does this represent (i.e., before, at, or after the equivalence point)? Support your answer with calculations and/or a logical explanation. What is the pH of this solution after the 50.0 mL of 0.200 M NaOH is added to the initial HF solution? Don't forget to account for dilution.

Explanation / Answer

1)

a) initial pH :

HF --------------------> H+ + F-

0.50 0 0

0.50-x x x

Ka = [H+][F-]/[HF]

6.8 x 10^-4 = x^2 / 0.5-x

x^2 + 6.8 x 10^-4 x - 3.4 x 10^-4 = 0

x = 0.018

[H+] = 0.018 M

pH = -log [H+]

pH = 1.74

b)

HF + NaOH ------------------------> NaF + H2O

c)

millimoles of HF = 25 x 0.500 = 12.5

millimoles of NaOH = 50 x 0.2 = 10

millimoles of HF > millimoles of NaOH . so it is before equivalence point. so it is buffer solution

d)

HF + NaOH --------------> NaF + H2O

12.5 10 0 0

2.5 0 10 10

pH = pKa + log [NaF]/[HF]

pH = 3.17 + log (10/2.5)

pH = 3.77

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