Question

At a particular temperature, 12.0 moles of so3 is placed into a 3.0-l rigid container, and the so3 dissociates by the reaction 2so3(g) 34 2so2(g) 1 o2(g) at equilibrium, 3.0 moles of so2 is present. calculate k for this reaction.

Answer 1

            2 SO₃ --> 2 SO₂ + O₂
I             12             0          0
C           -2x           +2x      +x
---------------------------------------------
E         12-2x          2x         x

Since the moles of SO₂ at equilibrium is 3 mol, 2x = 3. Then, x = 1.5 mol. So, the amounts at equilibrium is:
SO₃: 12 - 2(1.5) = 9
SO₂: 2(1.5) = 3
O₂: 1.5

The formula for K basing on the stoichiometric reaction is:
K = [SO₂]²[O₂]/[SO₃]² 
where the unit used is conc in mol/L.

K = [3 mol/3 L]²[1.5 mol/3 L]/[9 mol/3 L]²
K = 0.0556