Equilibrium

1.9+1.9 = 3.8 moles
There was 3 moles of HI, so 3.8-3= 0.8 moles left for H2+I2 (so half it.. 0.4 moles of H2 and 0.4 moles of I2 at equilibrium)

Moles = conc*Vol so Conc = Moles/Volume

250cm3 Volume - convert this to dm3 (250/1000) = 0.25dm3

Conc of HI:
3/0.25 = 12M
Conc of H2:
0.4/0.25 = 1.6M
Conc of I2: 1.6M

Kc = Products/Reactants
Kc = [HI]^2/

[I2] (we square the HI conc as we have 2 moles)
Kc = [144]/[1.6][1.6]
Kc = [144]/[1.6x1.6]
Kc = 144/2.56 = 56.25

Same as above yup
For units:
We use concentration to find Kc... conc is moldm-3
Kc = [HI]^2/[I2]

Kc = [moldm-3]^2/[moldm-3][moldm-3]
Kc = mol2dm-6/[moldm-3][moldm-3]
They cancel each other out, so there are no units for Kc

Okay, 1.9 + 1.9 moles were put in, making 3.8 moles altogether.

So if 3.0 moles ended up as HI then the other 0.8 must be H2 and I2 (so 0.4 of each)

Converting this to concentration in mol dm-3 by dividing by the volume (0.25 dm3) gives 12 for [HI], 1.6 for

and 1.6 for [I2].

Therefore Kc = ([HI]^2) / (

[I2]) = (12^2) / (1.6*1.6) = 144/2.56 = 56.25 arbitrary units

I know that's not the answer you gave but I think that answer is wrong, the units definitely are because the Kc should be unitless in this case

I'm sooo sorry but I wrote the wrong question, thats why neither of you got 6.6 would you please help me with this one.

In an experiment, 9.0 moles of nitrogen and 27 moles of hydrogen were placed into a vessel of volume 10 dm3 and allowed to reach equilibrium. It was found that two thirds of the nitrogen and hydrogen were converted into ammonia. Calculate Kc for the reaction.
N2(g) + 3H2(g) == 2NH3(g)

It's the same principal!

So there was 1/3 mole of N2 and H2 left at equilibrium
1/3 of 9 = 3
1/3 of 27 = 9

So 6 moles N2 reacted to produce ammonia. Using mole ratio 1:2, 6x2 = 12 moles ammonia made

Kc = [NH3]^2 / [N2]

^3
[NH3] = 12/10 = 1.2
[N2] = 3/10 = 0.3

= 9/10 = 0.9

Kc = [1.2]^2 / [0.3][0.9]^3 = 6.58 = 6.6 (1 dp rounded)

Thanks alot. I know this is a really basic/stupid question but how do you know to use the number of moles N2 to work out the no. of moles of ammonia. How do you know not to use the no. of moles of H2

Ok so you have 18 moles H2 and 6 moles N2 reacting

N2(g) + 3H2(g) == 2NH3(g)

For N2... 1:2 ratio so we have 12 moles NH3
For H2... 3:2 ratio so we have (18/3)*2 = 6*2 = 12 moles NH3
You can use either way. What is important to remember is that they both react TOGETHER to form ammonia so you don't need to use the ratios separately then add them together