Question

Question 1

0.11 M NaCN; Kf for Ag(CN)−2 is 3.0×1020.

Express your answers using two significant figures.

Question 2

What is ΔG for the formation of solid uranium hexafluoride from uranium and fluorine at 25∘C when the partial pressure of F2 is 0.077 atm ? The standard free energy of formation of UF6(s) is -2068 kJ/mol.
U(s)+3F2(g)→UF6(s)

Express your answer to four significant figures and include the appropriate units.

Question 3

Calculate the equilibrium constant Kp at 25 ∘C for the following reaction:
2SO2(g)+O2(g)→2SO3(g), ΔG∘ = -141.8 kJ

Express your answer using two significant figures.

Question 4

Given values of ΔG∘f at 25 ∘C for liquid ethanol (-174.9 kJ/mol) and gaseous ethanol (- 167.9 kJ/mol), calculate the vapor pressure of ethanol at 25 ∘C.

Express your answer using two significant figures.

Answer 1

Given that

Kf forAg(CN)−2 = 3.0×10^20

Ksp = 1.5 × 10⁻¹⁶
Ans 1

Ag⁺(aq) + 2 CN⁻(aq) ⇌ Ag(CN)₂⁻

Kf = [Ag(CN)₂⁻]/[Ag⁺][CN⁻]² = 3.0 × 10²⁰

But to get the solubility of AgI in this solution, you need the equilibrium constant for the reaction
AgI(s) + 2 CN⁻(aq) ⇌ Ag(CN)₂⁻ + I⁻

K = [Ag(CN)₂⁻][I⁻]/[CN⁻]² = ?

K = [Ag(CN)₂⁻][I⁻]/[CN⁻]² = {[Ag(CN)₂⁻]/[Ag⁺][CN⁻]²} • {[Ag⁺][I⁻]}
K = Kf × Ksp = (3.0 × 10²⁰) × (1.5 × 10⁻¹⁶) = 4.5 × 10⁴

Thus,

in 0.11 M NaCN, [CN⁻] = 0.11 M (initially)
In general, [CN⁻] = 0.11 – 2x
where, in this case, [Ag(CN)₂⁻] = [I⁻] = x

x² = K × [CN⁻]² = (4.5 × 10⁴)(0.11 – 2x)²

544.5 + 180000x² - 19800x - x² = 0
Solve for x ⇒

x = 0.05587 = [Ag(CN)₂⁻] = molar solubility

Basically, this means that AgI will dissolve until almost all the cyanide ion is used up.