NEED HELP with KC equilibrium question please!

referring to JAN 2012 AQA unit 4 question 2
http://store.aqa.org.uk/qual/gce/pdf/AQA-CHEM4-QP-JAN12.PDF
Need help with 2ai and 2aiv please

At high temperatures and in the presence of a catalyst, sulfur trioxide decomposes according to the following equation.

2SO3(g) 2SO2(g) + O2(g) ΔH = +196 kJ mol–1

2 (a) In an experiment, 8.0mol of sulfur trioxide were placed in a container of volume 12.0 dm3 and allowed to come to equilibrium.
At temperature T1 there were 1.4mol of oxygen in the equilibrium mixture.

2 (a) (i) Calculate the amount, in moles, of sulfur trioxide and of sulfur dioxide in the equilibrium
mixture.
Amount of sulfur trioxide - Answer is 5.2 - but i get 3.8 - i minus 1.4 and 2.8 from the initial moles of so3
Amount of sulfur dioxide -i got 2.8
(2 marks)
2 (a) (ii) Write an expression for the equilibrium constant, Kc, for this equilibrium.
kc = [so2]^2[o2]/[so3]^2

(1 mark)
2 (a) (iii) Deduce the units of Kc for this equilibrium.

i got mol dm^-3
(1 mark)

2 (a) (iv) Calculate a value of Kc for this equilibrium at temperature T1
(If you were unable to complete the calculations in part (a) (i) you should assume that
the amount of sulfur trioxide in the equilibrium mixture was 5.8mol and the amount of
sulfur dioxide was 2.1mol. These are not the correct values.)

Here is where i get really stuck
i understand I have to conver the moles into concentrations so i can use it in the KC formula derived earlier.
according to the mark scheme I divide the moles of o2, so3 and so2 by 12 dm^3. I don't know why you're supposed to divide each moles by 12. I thought they were gases so you had to first convert 12dm^3 into m^3 by dividing it by 1000. :confused: