Equilibrium Constant Table Method

[TheRedDevil18]

Hi, I just want someone to explain this table to me please, I am totally confused especially with the Change(mol).
Question below:

 

[mfb]

If 1 Mole of NH3 is produced, how many moles of N2 and H2 were used to do that?
Hint: Use the molar ratio.

If you know the initial quantity and the change, you get the quantity at equilibrium.

 

[TheRedDevil18]

How?, still a bit confused. Could you show me?

 

[mfb]

Is "how" related to the quantity at equilibrium?
This is a simple addition. If you start with X moles and produce Y moles, you have X+Y moles.

Is "how" related to the other part?
Just keep in mind that the total number of atoms (both for N and H) cannot change.

 

[TheRedDevil18]

I mean the Change, how do they get the change?

 

[mfb]

To produce 2 molecules of NH3, you need 1 molecule of N2 (to have 2 N atoms).
To produce 1 molecule of NH3, you need 1/2 molecule of N2.
To produce 1 mole of NH3 (this is given), you need 1/2 moles of N2.

It is that easy.

 

[TheRedDevil18]

Okay I understand it now, mole ratio N2:NH3 1/2*1=0.5 and H2:NH3 3/2*1=1.5, the signs are negative because they are being used up right?

 

[mfb]

Correct.

 

[TheRedDevil18]

What would happen if the change is negative for NH3, Would it now mean that NH3 is being reversed back into its reactants and the reactants would be positive?

 

[mfb]

You would need some initial NH3 for that.
In that case: Right.

 

[TheRedDevil18]

Okay I think that's it, Thanks for your help