This is a missing part of your question: The equilibrium system between sulfur dioxide gas, oxygen gas, and sulfur trioxide gas is given. So you need the equilibrium balanced equation of SO2, O2, SO3 reaction: First, we will start with the original equation which is not balanced yet (to understand how we get it): SO2 + O2 ↔ SO3 Here the number of O atom is not equal at the to sides So we will start to balance our equation by make the number of O atom equal each other on both sides: So we will start to put 2SO3 instead of SO3 and put 2SO2 instead of SO2 to balance also the S atom on both sides So we will get this: 2SO2(g) + O2(g) ↔ 2SO3(g) (This is our equilibrium balanced equation) know we have a number of O atom equals on each side = 6 and the sulfur equals on each side = 2
