A mixture of 1 mole of H₂O and 1 mole of CO is taken in a 10 litre container and heated to 725 K. At equilibrium 40% of water by mass reacts with carbon monoxide according to the equation :
CO(g)+H₂O(g)⇌ CO₂( g)+H₂( g).
The equilibrium constant K_c× 10² for the reaction is ____________. (Nearest integer)

Solution

Chemistry ma’am · 3 steps

Turn 40% by mass into moles first. Then the 10 L cancels, because both sides have 2 moles of gas.

1. Mass to moles. A fixed fraction of the mass is the same fraction of the moles. So 40% of 1 mol of H₂O reacting means x = 0.4 mol reacts.

   Moles	CO	H₂O	CO₂	H₂
   Initial	1	1	0	0
   Change	-0.4	-0.4	+0.4	+0.4
   Equilibrium	0.6	0.6	0.4	0.4

2. Volume cancels. Each concentration is moles divided by 10 L. There are two concentration terms on top and two below, so the factors of 10 cancel and you can use moles directly.

   K_c = [CO₂][H₂]/[CO][H₂O] = (0.4 × 0.4)/(0.6 × 0.6) = 0.16/0.36 = 0.444

3. So. K_c × 10² = 44.4, which rounds to 44.

The common slip: 225. Writing reactants over products gives K_c = 2.25, which becomes 225. Products always go on top.

The idea to keep. When the moles of gas are the same on both sides, the volume cancels in K_c. Build the ICE table in moles and plug the moles straight in.

Answer from the NTA final answer key. Solution by PrepWiser.