I've been having trouble understanding this

Question

In the reaction below, initially 5.0 atm of A and an unknown pressure of B are present in a flask at a temperature of 298 K.

2A (g) + B (g) ↔ 2C (g) ΔG = -12.5 kJ/mol

(a) Write the expression for the equilibrium constant, K, for the reaction represented above.

(b) Determine the value of K for this reaction at 298 K.

(c) At equilibrium, 92% of A had been converted to product. Determine the equilibrium partial pressure of each gas in the reaction? 

(d) Determine the initial partial pressure of gas B in the reaction.

Answer 1

a) K = [C]² / [A]²[B]

b) ∆G = -RT ln K

-12.5 kJ/mol = - (0.008314 kJ/molK)(298K) ln K

ln K = 5.05

K = 156

c) 2A + B ==> 2C

....5.........x.............0.......initial

5-4.6.....-2.3.........+4.6....Change

0.4......x-2.3..........4.6.....Equilibrium

K = (4.6)2 / (x-2.3)(0.4)2 = 156

Solve for x = 3.16 atm

Equilibrium pressure of A = 0.4 atm

Equilibrium pressure of B = 0.86 atm

Equilibrium pressure of C = 4.6 atm

Original pressure of B = 3.16 atm