Answer to Question #133505 in General Chemistry for hi

Question

Calculate the pH and degree of dissociation of:-
a. 0.8M HF
b. The same solution which is also in 0.1M NaF, If ka=6.8x10-4.

Answer 1

HF --> H⁺ + F^-

a) K_a ≈ [H⁺][F^-]/[HF]

[H⁺] = [F^-]

[H⁺]² = K_a[HF] = 6.8*10^-4 * 0.8 = 5.44*10^-4

[H⁺] = 2.3*10^-2 M

pH = -log [H⁺] = 1.64

$K_a=\alpha^2C/(1-\alpha)$

6.8*10^-4 = $(\alpha^2*0.8)/(1-\alpha)$

$\alpha=0.029$ (or 2.9 %)

b) For buffer solutions

pH = pK_a + log [salt]/[acid]

Therefore,

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

pK_a = -log K_a

pH = -log K_a + log [NaF]/[HF] = 3.17 + log (0.1 / 0.8) = 3.17 - 0.90 = 2.27

Answer:

$\alpha=0.029$ (or 2.9 %)

a) pH = 1.64

b) pH = 2.27