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# Chemistry GCSE percentage yield calculation

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1. Hi,
Could anybody look at this question and tell me where I am going wrong please?

In a lime kiln, calcium carbonate is decomposed to calcium oxide.

CaCO3 - CaO + CO2

1500 tonnes of calcium carbonate gave 804 tonnes of calcium oxide. Calculate the percentage yield for the process.

(Ar values: C = 12, O = 16, Ca = 40)

I worked out Ar values to be 100 for calcium carbonate and 56 for calcium oxide

Now, the way I thought to work it out is amount of product produced divided by maximum amount of product possible x 100, so:

804 / 1500 x100 but apparently this answer is incorrect.

Can somebody tell me where I am going wrong please?
2. I find it easier to think of in terms of moles

moles = mass/Mr
so moles of calcium carbonate = 1500/100 = 15 (note that his isn't the actual number of moles as we're dealing in tonnes but the difference cancels out so we will ignore it)
there is a 1:1 ratio so we expect 15 moles of calcium oxide as well
actual moles of calcium oxide = 804/56 = 14.36

14.36*100/15 = 95.7%

Your mistake was thinking that 1500 tonnes of CaCO3 -> 1500 tonnes of CaO but this is not how it works. You need to go through moles for this (since some of the mass is lost to the CO2)
3. (Original post by dfbenjamin)
I find it easier to think of in terms of moles

moles = mass/Mr
so moles of calcium carbonate = 1500/100 = 15 (note that his isn't the actual number of moles as we're dealing in tonnes but the difference cancels out so we will ignore it)
there is a 1:1 ratio so we expect 15 moles of calcium oxide as well
actual moles of calcium oxide = 804/56 = 14.36

14.36*100/15 = 95.7%

Your mistake was thinking that 1500 tonnes of CaCO3 -> 1500 tonnes of CaO but this is not how it works. You need to go through moles for this (since some of the mass is lost to the CO2)
Thank you! That makes much more sense now.

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