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# Calculating Kp

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1. Need help with question 6 as I have only 0.25 and 0.75 as the initial moles of the reactants due to the 1:3 ratio. However, the moles are just 1 and 3. Why is that because I thought the ratio add up to 100%.

Thanks
2. What do you mean by "the moles are just 1 and 3"?
3. (Original post by alow)
What do you mean by "the moles are just 1 and 3"?
the question wants you to assume that the initial moles the reactants are 1 mole and 3 moles. Thanks
4. (Original post by coconut64)
the question wants you to assume that the initial moles the reactants are 1 mole and 3 moles. Thanks
Does it? It only gives a ratio.
5. (Original post by alow)
Does it? It only gives a ratio.
That's what I thought but here is the explanation: 'Assuming no product gases (eg NH3) are there to start with (the question hintsat this by saying “left to equilibrate”) we can say that the initial mixture consists of 1 mol of N2(g) and 3 mol of H2(g) as this is the initial ratio'
6. (Original post by coconut64)
That's what I thought but here is the explanation: 'Assuming no product gases (eg NH3) are there to start with (the question hintsat this by saying “left to equilibrate”) we can say that the initial mixture consists of 1 mol of N2(g) and 3 mol of H2(g) as this is the initial ratio'
Yeah that's because it's the easiest amounts to deal with. You can use any amount you want though, as long as the 1:3 ratio holds.

You could use 0.0073 mol Nitrogen and 0.0219 mol Hydrogen if you want. That would be stupid, but it would get you the same answer.
7. (Original post by alow)
Yeah that's because it's the easiest amounts to deal with. You can use any amount you want though, as long as the 1:3 ratio holds.

You could use 0.0073 mol Nitrogen and 0.0219 mol Hydrogen if you want. That would be stupid, but it would get you the same answer.
But if I am using this to work out Kp, it will affect the end result since I used 0.25 and 0.75, which are smaller than 1 and 3.
8. (Original post by coconut64)
But if I am using this to work out Kp, it will affect the end result since I used 0.25 and 0.75, which are smaller than 1 and 3.
But your amount of produced ammonia will also be lower to account for that.
9. (Original post by alow)
But your amount of produced ammonia will also be lower to account for that.
The question says that 15% of ammonia is produced, can I just say that it is 0.15 mol?
10. (Original post by coconut64)
The question says that 15% of ammonia is produced, can I just say that it is 0.15 mol?
No, write down the equation for production of ammonia from hydrogen and nitrogen gases.

0.15mol cannot be 15% of the total.
11. You can start with any amount of hydrogen and nitrogen as long as it's a 1:3 ratio. So 1mol of hydrogen and 3mol of nitrogen or 271.5mol of hydrogen and 814.5mol of nitrogen! It does not matter
12. (Original post by Tuffyandtab)
You can start with any amount of hydrogen and nitrogen as long as it's a 1:3 ratio. So 1mol of hydrogen and 3mol of nitrogen or 271.5mol of hydrogen and 814.5mol of nitrogen! It does not matter
I used 0.25 and 0.75 as my 1:3 ratio. I though for ammonia you just find out 15% of 1 which is 0.15. Thanks
13. (Original post by alow)
No, write down the equation for production of ammonia from hydrogen and nitrogen gases.

0.15mol cannot be 15% of the total.
So the equation would be N2+ 3H2 -> 2NH3
I'm not sure where to go from here...
14. (Original post by coconut64)
So the equation would be N2+ 3H2 -> 2NH3
I'm not sure where to go from here...
You need to notice that when you react 4 moles of gas together, you get 2 back out.

This means your total amount will decrease as the reaction goes to equilibrium. So there won't be 0.15mol of ammonia as 0.15mol will be more than 15%.
15. (Original post by alow)
You need to notice that when you react 4 moles of gas together, you get 2 back out.

This means your total amount will decrease as the reaction goes to equilibrium. So there won't be 0.15mol of ammonia as 0.15mol will be more than 15%.
Actually, I need a min
16. I agree i think but I haven't done this topic before so bare with (I'm going onto 2nd year a level chemistry).
n2 + 3h2 --> 2nh3
mole fraction of nh3 = 0.15
mole fraction of n2 = 0.25 - (0.15/4) = 0.2125
mole fraction of h2 = 0.75 - 3(0.15/4) = 0.6375
Pa of nh3 = 1500
Pa of n2 = 2125
Pa of h2 = 6375
Kp = (1500)^2/(2125) x (6375)^3 = 4.08679653e-9 I hope this helps. Happy to explain

(Original post by coconut64)
I used 0.25 and 0.75 as my 1:3 ratio. I though for ammonia you just find out 15% of 1 which is 0.15. Thanks
17. (Original post by coconut64)
Actually, I need a min
Does it consider the change in amounts? Where do they get 6 moles of gas from?

Look at the second question here and its answer, it's basically the same thing but they don't give you a final answer as all of the pressures at equilibrium are unknown:

http://chem.libretexts.org/Core/Phys...tial_Pressures
18. (Original post by alow)
Does it consider the change in amounts? Where do they get 6 moles of gas from?

Look at the second question here and its answer, it's basically the same thing but they don't give you a final answer as all of the pressures at equilibrium are unknown:

http://chem.libretexts.org/Core/Phys...tial_Pressures
But that question only gives you the partial pressure whereas the question I'm doing now gives you hints about the inital moles and moles at equilibrium here is the explanation
19. (Original post by coconut64)
But that question only gives you the partial pressure whereas the question I'm doing now gives you hints about the inital moles and moles at equilibrium
You know there's a 1:3 ratio at the beginning and you know the pressure is 10000kPa. Use this information to draw a table like the one in the example.
20. (Original post by alow)
You know there's a 1:3 ratio at the beginning and you know the pressure is 10000kPa. Use this information to draw a table like the one in the example.
I have actually done that already but I don't know how to work out the mole of ammonia so my equilibrium moles for the reactants are wrong. If I am using the values 0.25 and 0.75 for my 1:3 ratio, it wouldn't work as 3/2 of 0.9 is 1.35

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