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# Chemistry Revision -Moles and their concepts

Chemistry discussion, revision, exam and homework help.

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1. Chemistry Revision -Moles and their concepts
Question:
A sample of Phosphoric (v) [H][/3]P[O][/4] acid contains 1.2*[10][/23] molecules. Calculate how many moles of Phosphoric (v) this is.

From my understanding of chemistry so far is the variations of moles but I am not sure whether I should be using the Moles = mass * mR.
If that's the case then it would be;
H = 1*3 = 3
P = 1*31 = 31
O= 16*4 = 64
Total : 98

How do I work out the mass, would I multiply it by 1.2*[10][/23] ?
Or is it the volume concentration? And if so? How would I work around that?
2. Re: Chemistry Revision -Moles and their concepts
(Original post by Tollybe)
Question:
A sample of Phosphoric (v) [H][/3]P[O][/4] acid contains 1.2*[10][/23] molecules. Calculate how many moles of Phosphoric (v) this is.

From my understanding of chemistry so far is the variations of moles but I am not sure whether I should be using the Moles = mass * mR.
If that's the case then it would be;
H = 1*3 = 3
P = 1*31 = 31
O= 16*4 = 64
Total : 98

How do I work out the mass, would I multiply it by 1.2*[10][/23] ?
Or is it the volume concentration? And if so? How would I work around that?
You have to use the Avogadro's Constant for this.

There are 6.02*10^23 particles in one mole of any substance.

Here you have 1.2*10^23 molecules.... How would you find the number of moles of H3PO4 ?

EDIT :

I realised that you're trying to use the formula moles = Mass/molar mass.

Note that it is mass divided by molar mass. The formula you have stated is wrong.

You cannot use this formula because, as you have pointed out you do not have the mass of the acid.

Also, if you were to multiply the no. of molecules by the molar mass of one molecule you would NOT obtain the mass of the acid but the molar mass of each and every molecule in the acid added together.

This is not the same as the mass of the acid that would be determined by weighing it, for example.
Last edited by Ari Ben Canaan; 19-04-2012 at 10:06.
3. Re: Chemistry Revision -Moles and their concepts
Thank you,
I divided it by Avogadro's constant.
Appreciated.
I'm really bad at the application of questions. Too many formulas!

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Last updated: May 18, 2012
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