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Help! Stuck on MCQ

Equation.
3H2+N2-------->2NH3
If 60 tonnes of hydrogen produces 80 tonnes of ammonia what is the percentage yield in the reaction?

Reply 1

alkyone

you can calculate what weight of ammonia you should obtain from 60tonnes of hydrogen, by calculating the number of moles of H2 in 60 tonnes and then multiplying by 2/3. Then your percentage yield is your answer divided by 80 times 100.

Reply 2

JOY33

Original post by alkyone

Since all are gases you can take mol ratio between H2 and NH3 to be 2:3 Isn't H2 limiting? because from 60 tonnes of H2 you get (40 tonnes of NH3) 2/3*60 NH3 which is 40 tonnes

Reply 3

alkyone

Original post by JOY33

Since all are gases you can take mol ratio between H2 and NH3 to be 2:3 Isn't H2 limiting? because from 60 tonnes of H2 you get (40 tonnes of NH3) 2/3*60 NH3 which is 40 tonnes

Moles is not the same as tonnes though. You need to convert to account for differences in the Molecular weight of the two.

Reply 4

JOY33

What? Whatever prefix you take weather it be Kg to mg you can directly take the mole ration of H2 to NH3 to be 2:3 directly from the equation since they are all in gaseous states

Reply 5

alkyone

Original post by JOY33

What? Whatever prefix you take weather it be Kg to mg you can directly take the mole ration of H2 to NH3 to be 2:3 directly from the equation since they are all in gaseous states

So for every 3 molecules of H2 you get 2 of NH3. But because they have different Mr: H2=2 and NH3= 17, that means that three molecules of H2 have an Mr of 6, and 2 Molecules of NH3 have an MR of 34. So mass-wise, NH3 is much heavier than H2 despite having more moles of H2. Which is why you need to convert first.

This is not about converting from mg to tonnes or kg, this is about converting from tonnes to moles, a completely different unit representing number of molecules rather than mass.

(edited 8 years ago)

Reply 6

JOY33

Is the answer 80/340*100

Reply 7

alkyone

Yeah that's what I get too

Reply 8

JOY33

What are the exact values you used to arrive at 340?
Because according to what you previously said theoretical yield was supposed to be 80

Reply 9

alkyone

I just used the ratio of total molecular weights: 6:34, so 60 tonnes give 340 tonnes of ammonia.

Reply 10

alkyone

So I did the opposite of what I said and instead of converting everything to moles, I converted moles to a weight ratio. The other way is:

1mole-->2 mg
So 60tonnes is 3x10^10 moles.
X 2/3= 2X10^10 moles of ammonia.
17X 2X10^10= 34 X10^10 mg=340 tonnes.