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# Help with this gcse chemistry question?

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1. The answer is 21.2% but i don't know how to get it?

Ammonia Gas (NH3) is made by heating the gases nitrogen and hydrogen under pressure in the prescence of an iron catalyst:
N2 + 3H2 ⇌ 2NH3
If 7.0g of nitrogen are reacted with excess hydrogen and 1.8g of ammonia is collected, what is the percentage yield?
2. To work out the percentage yield you have to do the (actual amount in moles divided by the theoretical amount in moles) * 100. To work out the actual amount in moles of NH3 we need to do mass / mr, so it is 1.8/17 which gives us 9/85mol. To find the theoretical amount in moles we need to find the moles of N2 and then equate this to 2NH3. So, moles of N2 = 7/28 which gives us 0.25mol and as it is a 1:2 ratio amount of moles of NH3 is 0.5mol. Now we put these values into our equation: ((9/85)/0.5)*100 which gives us 21.1764.....% which is 21.2% to 3 significant figures.
3. (Original post by hamzahussain97)
To work out the percentage yield you have to do the (actual amount in moles divided by the theoretical amount in moles) * 100. To work out the actual amount in moles of NH3 we need to do mass / mr, so it is 1.8/17 which gives us 9/85mol. To find the theoretical amount in moles we need to find the moles of N2 and then equate this to 2NH3. So, moles of N2 = 7/28 which gives us 0.25mol and as it is a 1:2 ratio amount of moles of NH3 is 0.5mol. Now we put these values into our equation: ((9/85)/0.5)*100 which gives us 21.1764.....% which is 21.2% to 3 significant figures.
Thank you so much for your help!

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