Aspirin C9H8O4 is made when salicylic acid C7H6O3 reacts with ethanoic anhydride C4H6O3 . The equation for this reaction is: C7H6O3 + C4H6O3 → C9H8O4 + CH3COOH Calculate the maximum mass of aspirin that could be made from 100 g of salicylic acid.

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A relevant equation is:

mass = moles x relative formula mass 

Also written:

m = n*Mr

This reaction is a 1:1 ratio of salicylic acid to aspirin, meaning 1 mole of salicylic acid makes a maximum of 1 mole of aspirin. We know this because in the equation the coefficients to C9H8Oand C7H6O3 are both 1. 

So let's find out the number of moles of salicylic acid there is in 100g.

Mr salicylic acid = 138 g mol-1 because:

It has 7 Carbon atoms at 12g mol-1 and 6 Hydrogen atoms at 1g mol-1 and 3 Oxygen atoms at 16g mol-1.

7*12 + 6*1 + 3*16 = 138g mol-1

m/Mr = 100/138 = 0.7246...mol of salicylic acid.

There will also be 0.7246... mol of aspirin.

The Mr of aspirin is 180g mol-1. This can be found from the same method used to find the Mr of salicylic acid.

mass of aspirin = n * Mr = 0.7246... * 180 = 130.435... g

This rounds to 130g to 3.s.f.

It is 3 significant figures because this is the lowest degree of accuracy we have used throughout the question.

NOTES

A common easy mistake is to round within the question, ie. rounding the number of moles created to 0.725 or worse to 0.72 or 0.7. This can cause errors later in the question. Avoid this error by using the exact values in your final calculation. So where we calculated the mass of aspirin with

mass = n * Mr, use n = 100/138 in your calculator as this is the exact value. 

Emma H. IB Chemistry tutor, GCSE Chemistry tutor, IB Physics tutor, G...

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