chemistry - equilibrium?

Question

At equilibrium, the value of the reaction quotient, Qp, is 0.636. What will be the value of Qp if the total pressure of the reaction shown below is doubled?

A (g) ↔ 4 B (g)

the answer is 5.09 but why?

Answer 1

5.09 = 0.636*8, so doubling the pressure increases Qp by a factor of 8. Qp normally depends on concentration. Doubling the pressure reduces volume to 0.5 times its original value, doubling the concentration. Reaction rate in a reversible reaction A <--> B is governed by Qp = [B]^x / [A]^y. Since [A] / [B] remains unchanged, it must be the case that x - y = 3 so that Qp depends on the cube of the concentration of B. If the concentration doubles, Qp increases by a factor of 8.