Equilibrium — Practice

Question 1

For H2 + I2 <=> 2HI, Kc = 54.8 at 425 C. If 0.5 M H2 and 0.5 M I2 are mixed, find [HI] at equilibrium.

Accepted Answer

54.8 = (2x)^2/((0.5-x)(0.5-x)). Square root: 7.4 = 2x/(0.5-x), so x = 0.393. [HI] = 2x = 0.786 M.

Question 2

Find the pH of a 0.1 M CH3COOH solution (Ka = 1.8e-5).

Accepted Answer

[H+] = sqrt(Ka x C) = sqrt(1.8e-5 x 0.1) = sqrt(1.8e-6) = 1.34e-3 M. pH = -log(1.34e-3) = 2.87.

Question 3

Find the pH of a buffer of 0.1 M CH3COOH and 0.2 M CH3COONa (pKa = 4.74).

Accepted Answer

pH = pKa + log([Salt]/[Acid]) = 4.74 + log(0.2/0.1) = 4.74 + 0.30 = 5.04.

Question 4

Ksp of BaSO4 is 1.5e-9. Will a precipitate form when 0.01 M BaCl2 and 0.01 M Na2SO4 are mixed?

Accepted Answer

IP = [Ba2+][SO4 2-] = (0.005)(0.005) after mixing equal volumes = 2.5e-5, which is >> Ksp (1.5e-9). So yes, BaSO4 precipitates. (Even without dilution, IP = 1e-4 >> Ksp.)

Question 5

Predict the effect of increasing temperature on N2 + 3H2 <=> 2NH3 (delta-H = -92.4 kJ).

Accepted Answer

The forward reaction is exothermic, so raising the temperature shifts the equilibrium in the endothermic (backward) direction, producing less NH3.

Practice — Equilibrium

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