when the balanced equation for this reaction is:
2SO3 → 2SO2 + O2 so, by using ICE table:
initial 0.391 0 0.125m
change -X +2X +X
Equ (0.391-X) 2X 0.125+X
when Keq = concentration of products / concentration of reactants
so, by substitution
4.34 = [2X]^2 * [0.125+X] / [0.391-X]^2 by solving for X
∴ X = 0.357 m
∴ the concentration of SO2 = 2x = 2 * 0.357
= 0.714 m