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# C5 Moles and Molar Mass watch

1. Hello guys,

I've got a few questions regarding the topic - Moles and Molar Mass:

1) Is the volume taken up by any substance: Number of moles in the substance * 24.
1 mole of any gas = 24dm^3 = 24 litres.
If not, then what does the formula actually tell you - the volume of what?

2) The mole is the unit of substance or the number of atoms present in a substance.
Atoms are too small to be counted individually so they we use moles as another way of measuring things, a mole consists of 6.022 * 10^23 atoms = Avogrado's constant.
Why will iron have less number of atoms present in a certain mass than the same amount of mass for lithuim?
For example, why will 10g of lithium have more atoms in it than 10g of iron. Also, how many atoms will be present in each mass?

Is it because iron has heavier atoms as it has a higher relative atomic mass. Also, therefore 10g of iron will have lower amount of moles which will mean the mass will have less atoms present

Is this calculation right? Number of atoms present in 10g of iron = 10g (mass) / 56g (molar mass) = 0.17857 (approximately) moles.
0.17857 * (6.022 * 10^23) = 1.07 * 10^23 atoms.

Also, I do not understand this theory of 'The relative atomic mass (or Ar) of an element is defined as the average mass of an atom of the element compared to 1/12th the mass of an atom of carbon-12.'

I understand how the relative atomic mass of an element is calculated by working out the percentage of the two isotopes and multiplying them by their atomic number, adding them up and dividing by 100. I understand that part but how is it related to the 'carbon-12' isotope?

E.g. Ar of Bromine = 50% 79(mass number) Br and 50% 81(mass number) Br.
So, Ar of Bromine = [ (79 * 50) + (81 * 50) ] / 100 = 80.

EDIT: Done some calculations and the answer is correct, but I don't get why is it compared to carbon 12, is it because the relative atomic mass of carbon 12 is exactly 12.00000u. Also, isn't it easier to work out ther relative atomic mass through the percentage of the isotopes method or the total number of protons and neutrons in the nucleus instead of the carbon-12 method shown below:

(56 / (6.022*10^23)) / ((1/12)*(12/(6.022*10^23)) = Relative atomic mass of iron
2. 1) 1 mol of any gas occupies 24 dm3 (assuming it is close to an ideal gas). It is not as you initially stated 1 mol of any substance.

2) Atoms have such a small mass it is (usually) silly to talk about their absolute masses, so instead we use their relative masses - that way we can easily measure out simple ratios of numbers of atoms (important for stoichiometry). If you knew that one atom was twice as heavy as another atom and you knew they reacted in a 1 to 1 ratio, then you could simply measure out twice as many grams of one that the other and you'd know that you'd have the same number of atoms, so they could react and there would be none left over.

Why use carbon? For historical reasons; scientists used hydrogen for a while and oxygen for a bit until they settled on carbon. But they had to pick one to relate all of the other to, so why not carbon? They stated that C-12 has a mass of 12, so 1/12th of that must be 1. That way hydrogen is 1x 1/12th of the mass of C-12 and oxygen is 16x 1/12th of the mass of C-12.
3. (Original post by Pigster)
1) 1 mol of any gas occupies 24 dm3 (assuming it is close to an ideal gas). It is not as you initially stated 1 mol of any substance.

2) Atoms have such a small mass it is (usually) silly to talk about their absolute masses, so instead we use their relative masses - that way we can easily measure out simple ratios of numbers of atoms (important for stoichiometry). If you knew that one atom was twice as heavy as another atom and you knew they reacted in a 1 to 1 ratio, then you could simply measure out twice as many grams of one that the other and you'd know that you'd have the same number of atoms, so they could react and there would be none left over.

Why use carbon? For historical reasons; scientists used hydrogen for a while and oxygen for a bit until they settled on carbon. But they had to pick one to relate all of the other to, so why not carbon? They stated that C-12 has a mass of 12, so 1/12th of that must be 1. That way hydrogen is 1x 1/12th of the mass of C-12 and oxygen is 16x 1/12th of the mass of C-12.
Oh so they had to find an element which had a mass of exactly a whole number - 12.0000 and that when you multiply it to 1/12 of carbon 12, it'll be the relative atomic mass as 1/12 mass of carbon 12 is exactly 1.

Posted from TSR Mobile
4. By definition C-12 has a mass of 12.00000000000000000000000000000 00000000000000000000000000000000 00000000000000 etc.

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