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why do I not use avogadro's number

Posted: Sat Apr 07, 2018 5:22 pm
by Alondra Juarez section 1E
I am confused in a problem that involves conversions to atoms. For example, Mass of an atom of beryllium is 1.50 X 10^(-26)kg. How many beryllium atoms are present in a beryllium film of mass 0.210 g.

I know the answer is 1.50 X 10^(-26)kg. * (1000 grams) =1.5 X10^(-23) and now I will have to divide .210 by 1.5 X10^(-23) and get 1.40 X10 ^(22) atoms but I'm not too sure why we did not use Avogadros number in this problem for the conversion?

Re: why do I not use avogadro's number

Posted: Sat Apr 07, 2018 8:17 pm
by Noah Carey 1G
You use Avogadro's number when you are working with moles because 1 mole is equal to approximately 6.022 X 10^(23) atoms/molecules/etc.. The question doesn't ask for anything related to moles. It's interesting because it's actually giving you another method to find how many atoms there are in Beryllium. If you work it out you can still get the same answer using Avogadro's number (you calculate the moles of this sample .210gBe/9.01g/molBe and then multiply that answer (in moles) by Avogadro's number and you receive the same answer 1.40 X 10^(22)), but because the question tells you the mass of one Be atom you should solve it by multiplying this and the number of total grams of the sample. I hope this helps!

Re: why do I not use avogadro's number

Posted: Sun Apr 08, 2018 11:10 am
by Chem_Mod
Yes. Avogadro's number is a constant which relates 1 mole to number of "objects" (atoms, molecules, etc.).

The problem statement already gives you mass per atom and you are given a mass, so you can get the answer without having to looking up the molar mass and convert moles to atoms.

Re: why do I not use avogadro's number

Posted: Sun Apr 08, 2018 11:42 am
by Jack Martinyan 1L
You can use avogadro's number to solve the problem; however, it would take longer and woudn't be as productive. In this case, it's much easier to just divide the mass of the sample by the mass of the individual atom to get the number of atoms since we already have both values:

mass(sample) / mass(atom) = amount of atoms

Re: why do I not use avogadro's number

Posted: Wed Apr 11, 2018 11:25 pm
by Maria Zamarripa 1L
So just for clarification, Avogadro's number is only used when you're given moles?

Re: why do I not use avogadro's number

Posted: Thu Apr 12, 2018 1:42 am
by breannasung_1K
Avogadro's number means 1 mol of something = 6.0221x10^23 molecules or atoms of something, so to say you only use Avogadro's number when you are given mols is not a correct.