# Chemistry Moles ... Help Please

Discussion in 'Personal' started by ResourceFinder, Apr 3, 2011.

1. ### ResourceFinder

N2 + 3H2 = 2NH3

How much ammonia could be made from 1000g of nitrogen

I get N = 14g so N2 = 28g and 3H2 = 6g so we get 34g ammonia

So I also get that that the ratio 28:34 = 1000:1214 and we should get 1214g of ammonia

My concern is ... is there a more "chemical" reason I sort of said we need one mole of N2 and one mole of 3H2 to get one mole of 2NH3 and that is why we need 6g of 3H2 but I am not sure this is the "correct" explanation

2. ### lapinroseLead commenter

I thought moles were biology not chemistry, or spies.

3. ### ResourceFinder

I knew I should have posted on science

4. ### lapinroseLead commenter

Sorry, couldn't resist it!! Now we're on to electronics with resistors!!

5. ### anon3372

Blimmin eck - you have totally and utterly lost me.

6. ### lapinroseLead commenter

I understood the moles, but whether they're spies or the ones that live in the ground I don't know.

27

8. ### anon3372

I can very vaguely remember that mole is summat about pure amount of summat and summat to do with carbon.
After that, I am lost. I didn't do O level Chemistry, I had to do Physics instead (and never understood Physics either - my son now explains it to me).

The chemical reason is that you have one mole of nitrogen and obtain two moles of ammonia.

You don't really need to calculate the number of moles of hydrogen.

This is how I worked it out, and how you would probably be asked to work it out in a science lesson:
N2 + 3H2 -> 2NH3

Step 1 - calculate the number of moles of nitrogen:
N2 moles = mass/Mr (where Mr = relative molecular mass)

N2 moles = 1000 g / 2(14) g mol-1 = 35.71

Step 2 - work out how many moles of ammonia are produced:
You get two moles of ammonia, for every mole of nitrogen used up. Therefore:

2(35.71) = 71.43 moles of NH3

Step 3 - convert the number of moles of ammonia to mass (g):
moles = mass/Mr

mass = moles*Mr

mass = 71.43 * (14 + 3) = 1214.29 g of NH3

Hope this helps! Any questions - just ask!

Yes indeed. A mole is an amount of a substance that contains exactly 6.02 * 10^23 atoms or molecules.

It is based upon atomic mass.

1 mole of carbon has a mass of 12 g.

All of the atomic masses are based upon carbon - i.e. 12.00 g of C is exactly 6.02 * 10^23 atoms.

1 hydrogen atom has 1/12 the mass of a carbon atom. Therefore 1 mole of hydrogen has a mass of 1 g. And so on!

RF, your answer is correct. This is how I do it...

use the number moles=mass/molecular weight equation.

So... n=m/mr for nitrogen n= 1000/28 = 35.7

Ratio of nitrogen is 1:2 so to find the number of moles of ammonia, you multiply your moles of nitrogen by 2, giving 71.4 moles of ammonia.

You can then rearrange the original equation so that mass= moles x molecular mass

mass = 71.4 x 17 = 1213.8 g ammonia

So basically, its all about comparing the moles and their respective weights.

Mimi

12. ### anon3372

LOL, it does help that I'm on a chem PGCE, and my lovely first placement mentor gave me a couple of AS lessons!!!

14. ### anon3372

I would LOVE to understand it.
But I must admit, it is well beyond me.
I am a mere linguist. I did get a B in Biology and mystifyingly a C in Physics O level, but I am a complete numpty as far as Science topics are concerned.
None of it makes sense to me, no matter how much I google and try to understand it.
I am only starting to understand Physics at 42 as my son is explaining it to me!

See, I envy people who can speak other languages! I can manage a small amount of french, anything else, no hope!!! I wish I could though, feel like a bit of a fraud when on holiday and I have to make gestures!

16. ### ResourceFinder

Thanks

I know the 1214 is correct but it is the explaining that I was unsure ... I did not want to give incorrect terminology

From yours
/28 because it is N2 ... yes?

But ratio of nitrogen = 1:2 ... I can see 1 lot of N2 on the left and 2N on the right ... is that where the ratio comes from (I was thrown by N2 on left and 2N on right ... I guess they are not the same and the left hand 2 was dealt with by the 28)

The 17 comes from NH3 = 14 + 1*3 ... yes?

17. ### BethannieNew commenter

I am a Science Graduate (Maths - and I can cope with basic physics....but no Chemistry, I was excused Chemistry at school on medical grounds.....but I followed the earlier explanation!) and I speak a couple of languages........but I would love to be more artistic. Maybe be able to play an instrument?...we're never satisfied are we!

18. ### anon3372

See, the grass is always greener.
I can pick up any language anywhere.
But all this Science stuff - well, you don't just get the chance to drop it into everyday conversation.
I would love to understand it all - I feel like a right eeejit sometimes, when my 11 year old is explaining it to me!
I still to this day do not understand why a balloon rises - I know what I have been told as to why it rises, but it makes no sense as I cannot see the gas! I just have to believe it is there. Some invisible thing with mass, etc.
This balloon thing drove my step-dad mad. We ended up agreeing that I would just learn it off by heart so that I could pass my O level.
I still don't understand it because I cannot see it!

Yes, N2 so 28.

With regards to the ratio, 1 "lot" of N2, can make 2 "lots" of NH3, with the right anmount of hydrogen.

This relates to the bonding. N2 is diatomic, just like hydrogen is, and oxygen. The 2N relates to the fact the diatomic molecule has split, and been replaced by 3 hydrogens, but that there are 2 molecules of NH3. (Just to clarify, the number in front of a formula relates to how many molecules there are of the substance. The smaller subscript numbers after, relate to how many are present within one molecule)

Spot on!!

Is that of better help? (sorry i confused you)

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