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What is the mole ?

  1. May 5, 2013 #1
    I've read a lot about the mole , avogadro's no. , molecular mass and such , but I can't seem to understand anything , and no , the wiki article wasn't helpful at all ! Put simply :
    1- I read that the mole is the molecular mass expressed in grams , for example an Oxygen atom has 16 as a mass no and 8 as an atomic no. , does that mean that 1 mole of an oxygen atom is equivalent to 16 gms ? And since the oxygen molecule has 2 atoms then does that mean that one mole of it contains "2*16" moles ?
    2- What is the difference between the mass of one mole and the molecular mass ?
    3- what does avogadro's number have to do with all of this ? (please answer me soon , thank you in advance)
  2. jcsd
  3. May 5, 2013 #2

    Philip Wood

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    Gold Member

    A mole of atoms contains the Avogadro number, NA, of atoms (6.022 x 1023 atoms). A mole of molecules contains the Avogadro number of molecules (6.022 x 1023 molecules). That's what a mole is.

    Why this weird number? It is because NA is the number of unified atomic mass units (number of u) in one gram. That's why it is that a mole of carbon atoms has a mass of 12 gram: a carbon atom has a mass of 12 u, so NA carbon atoms must have a mass of 12 gram, because 12 gram is NA x 12 u. And so on.
  4. May 5, 2013 #3
    Thank you , this helped me define the mole
  5. May 5, 2013 #4


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  6. May 5, 2013 #5
    Thank you ! that was hilarious and helpful at the same time !
  7. May 5, 2013 #6
    One mole of a gas occupies 22.4 liters of space (at STP) and represents Avogadro's no. of MOLECULES - Therefore 22.4 liters of O2 would have ONE Avogadro's no. of molecules, or TWO Avogadro's no. of atoms.
    So if, for instance, you had 1.2 moles of O2 then you'd have 1.2 Avogadro's of molecules, or 2.4 Avogadro's of atoms. And since the atomic mass of O2 is 32 you'd also have 32 X 1.2 = 38.4 grams of mass.
    (The atomic mass of O=16, but remember you're dealing with the molecule O2, with an atomic mass of 32)

    If it were 2.6 moles of Tritium gas (H3) for instance, then you'd have 2.6 Avogadro's of H3 molecules, or 3 X 2.6= 7.8 Avogadro's of H3 atoms. And since the atomic mass of H3 is 3, you'd also have 3 X 2.6 = 7.8 grams of mass.

    Avogadro's number is useful for, and normally applied to pure and blended GASSES in Chem. 101 lessons. But it has real-world use in vapors and solids as well.
  8. May 6, 2013 #7


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    Staff: Mentor

    You have no idea what tritium is, don't you?

    Tritium is an isotope of hydrogen, with the nucleus made of one proton and two neutrons. It is not H3 (nor H3), if anything we would write it as 3H. And tritium molecule is 3H2 and has molar mass of 6 (as atomic mass of tritium is 3).
  9. May 6, 2013 #8
    From when i learnt chemistry, I remember that "a mole is Avogadro's number of anything" e.g. a mole of bananas would be 6.022x10[itex]^{23}[/itex] bananas. You wouldnt normally use the mole in this way (unless you had that many bananas maybe) but it helped me to get the idea.

    The other thing I remember was the number of moles N = weight (in grams) / formula weight (in atomic mass units). which i remembered as N = weight / formula weight.

    "formula weight" is an older term similar to "molecular mass" or "molar mass" but i think its easier to get my head around what it is since it doesnt have the word "mole" in it, so you dont have two or more terms containing the new word "mole" (or molar or molecular etc) that mean completely different things.

    the formula weight (= molecular mass = molar mass, etc) is the number of "atomic mass units" that something weighs. these are written on the periodic table, eg H is about 1, He is about 4, C is 12 and O is 16, etc. So the molecule H2O weighs 18 atomic mass units (2x1 for H +16 for the O).

    So a mole of water would weigh 18 grams , by rearranging the N = W/FW formula.

    The fractional weights on the periodic table represent actual numbers of real samples of the substance, which are fractions because in real life H also contains very small quantities of heavy hydrogen which weighs 2, which brings up the average weight of a "H" atom to 1.008. there are other reasons for the fractional atomic weights but when you are beginning you can think of them as having whole number weights and it will make more sense (then, the weight number is basically the sum of the number of protons plus neutrons that the atom has).

    by rearranging the above simple formula you can work out most basic questions about moles
    Last edited: May 6, 2013
  10. May 12, 2013 #9
    Thank you so much ! That clarified the whole idea to me of NA for molecules and atoms , it was extremely helpful !
    Last edited: May 12, 2013
  11. May 12, 2013 #10
    thank you for clarifying the molecular formula of tritium (is it called a molecular formula , I can't seem to remeber anything in chemistry!)
  12. May 12, 2013 #11
    Thanks a million , really , you don't know how much this has helped me , especially the formula without the word mole :D and the banana mole , thanks again
    Last edited: May 12, 2013
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