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# Mr & Co-efficients ???? watch

1. Hi all ,
This is a really stupid question but it's a trap I always fall into .I need to be sure.

Let's say I have the following reaction ;

2A + 3B -->> C + D and the mass of A is 10g.To work out number of moles I know I'll use n=m/Mr
For the Mr I don't multiply the co-efficient and all the individual masses of the elements in A right?

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2. If you have 10 g of A, it doesn't matter what you react it with, the number of particles will be the same, hence the number of moles of A you have is unaffected.

So, you're correct.
3. Nope, if A is an element, just the mass number and if a compound, add the mass numbers together. No multiplying to be done.
2 moles would be the theoretical amount, you're calculating the actual number of moles.

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Updated: January 18, 2015
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