Question

1. What is the concentration of NO gas at equilibrium if you mix 0.20 mol of N2 and 0.15 mol of O2 in a 1.0 L container at 2000 °C? The Kc for the reaction at 2000 °C is .
N2 + O2 -> 2NO

2. What would be the equilibrium pH if 200 milligrams of Hydrofluoric acid (HF) were dissolved in 1 liter of solution? The pKa for the acid is equal to 3.2. (Hint: Convert pKa to Ka)
HF <---> H+ + F-

Answer 1

Question #61309 – Chemistry – General Chemistry

Question

1. What is the concentration of NO gas at equilibrium if you mix 0.20 mol of N2 and 0.15 mol of O2 in a 1.0 L container at 2000 °C? The Kc for the reaction at 2000 °C is .

N2 + O2 -> 2NO

Solution

Solution is difficult because $K_c$ is not given by customer.

$$K_c = [NO]^2 / ([N_2][O_2])$$

If x mol of N2 react with O2

$$K_c = [2x]^2 / [0.2 - x][0.15 - x]$$ $$(K_c - 4)^* x^2 - 0.35 K_c x + K_c * 0.03 = 0$$ $$x = (4 - K_c + (0.35^2 - 4(K_c - 4)^* K_c * 0.03)^{0.5}) / (2(K_c - 4)) - \text{typical solution of quadratic equation}$$ $$[NO] = 2x = 2 * (4 - K_c + (0.35^2 - 4(K_c - 4)^* K_c * 0.03)^{0.5}) / (2(K_c - 4))$$

Answer: $[NO] = 2 * (4 - K_c + (0.35^2 - 4(K_c - 4)^* K_c * 0.03)^{0.5}) / (2(K_c - 4))$

Question:

2. What would be the equilibrium pH if 200 milligrams of Hydrofluoric acid (HF) were dissolved in 1 liter of solution? The pKa for the acid is equal to 3.2. (Hint: Convert pKa to Ka)

Solution:

HF <---> H+ + F-

$$[H^+] = [F^-] + [OH^-]$$ $$C_{HF} = [HF] + [F^-]$$ $$K_a = [H^+][F^-] / [HF]$$ $$[H^+] = K_a C_{HF} / (H^+ + K_a) + K_w / [H^+]$$ $$C_{HF} = 0.2 / (20*1) = 0.01 \, \text{mol/l}$$ $$K_a = 10^{-3.2} = 6.3*10^{-4}$$ $$[H^+] = 0.00222 \, \text{mol/l}$$ $$pH = 2.65$$

Answer: $pH = 2.65$

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