Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Moles - Why do chemists use them?

  1. Feb 1, 2010 #1
    1. The problem statement, all variables and given/known data
    Could you please explain to me what moles are and how it refers to Avogadro's Constant and why chemists use them? Thanks.

    2. Relevant equations
    There are no equations to my knowledge..

    3. The attempt at a solution
    I know what moles are I think but I am not sure how it refers to Avogadro's Constant and why a specialized chemist would use them.

    Please help and I will do my best to offer some too.

  2. jcsd
  3. Feb 1, 2010 #2


    User Avatar
    Science Advisor

    The mass, in grams, of a mole of a given compound is equal to the total molecular weight opf the compound in atomic units- thus, easy to calculate.
  4. Feb 1, 2010 #3
    Could you please explain to me how it refers to Avogadro's Constant? :\
  5. Feb 1, 2010 #4
    Basically, say I was asked to explain what moles are and how they refer to the Avogadro's Constant and why chemists use them.

    I hope that makes more sense to how I need help.

    Thanks guys.
  6. Feb 1, 2010 #5


    User Avatar
    Science Advisor
    Gold Member

    Recall that atomic mass units are defined to coincide with the number of nucleons (neutrons and protons) in an atom. The problem is that with E=mc^2 we don't quite get nice integer units due to the nuclear binding energies and also slight difference between proton and neutron free masses. However they are close to integer valued with the calibrating definition given as: Carbon 12 has exactly atomic mass of 12 units (AMU's).

    Of course AMUs are too small for practical work so it is nice to translate to practical units like grams.

    So the definition of a mole is that 12 grams of Carbon 12 is exactly one mole of Carbon 12. Or 1 gram = Avogadro's constant times 1 mass unit. Avagadro's number is how many carbon 12 atoms there are in 12 grams of carbon 12.

    Working with moles you can then use the chemical formulas of reactions since there is a 1-1 correspondence to number of moles and number of molecules (a la Avogadro's constant). Thus 2 moles of hydrogen gas (H2) plus 1 mole of oxygen gas (O2) yields 4 moles of atomic hydrogen and 2 moles of atomic oxygen which reacts to form 2 moles of water (H2O).
  7. Feb 1, 2010 #6
    Thanks for the information.


    Does anyone else have any more simpler information?

  8. Feb 1, 2010 #7


    User Avatar
    Science Advisor
    Homework Helper

    In chemistry you are thinking of individual atoms
    ie. water is one oxygen atom and two hydrogen atoms

    Avagadro's constant simply tells you how many atoms you have in so many grams of each substance
  9. Feb 1, 2010 #8
    I think there are many examples where the use of the mol makes calculations etc easier.As an example with the ideal gas equation we could refer to one kg of gas or one mol of gas.With one kg different gases have different constants but with one mol the constant is the same for all gases.
  10. Feb 1, 2010 #9
    Could you please give me some examples of Moles?
    Could you also please show them so I can further understand it?

  11. Feb 1, 2010 #10
    Do you know what I mean? If not, feel free to ask.
  12. Feb 1, 2010 #11
    A mole is just a number, like a dozen. Previous posts have already given information on how/why the number was chosen.
    Avogadro's constant, 6.0221415×1023 mol-1, just says that there are 6.0221415×1023 of something per mole of that something, much like a "dozen constant," 1 doz-1, would be 12 of something per dozen.

    Do you want a better explanation on why we use the number 6.0221415×1023 from the whole gram-amu Carbon 12 definition?
  13. Feb 1, 2010 #12
    I have understood a lot from thus forums so far but my final section of homework in relation to Moles tells me the following; "Examples of Moles are shown"

    In my notes, I have wrote something down like "10 moles of Argon gas; (40) + (0) = 40 | 1 mole = 40g x 10 = 400g"

    Is this what it is asking me to write down as examples? I don't get it :\
    Last edited: Feb 1, 2010
  14. Feb 2, 2010 #13
    This is confusing me lol.
  15. Feb 2, 2010 #14


    User Avatar

    As said above a mole is a number of atoms, molecules or ions (for starters). We know that O plus H eqals H20. So 1 O atom plus 2 H atoms equal 1 H20 molecule. If we take 1 mole of O 2 of H we get 1 of H20. Moles are better than weight, that would be 16 grams of O react with 2 grams of H to result in 18 g of water. Things are easy with simple molecules, but when you work with complex organic molecules or polymers weight is computational dead weight. Moles are the future!!!

    Historically the number was chosen as the number of atoms in 12g of C (be it diamond, graphite or else) , the exact number wasn't found out with certainty until much later, but it did'nt matter that much as the concept helped a lot in chemistry. It is usually useless to know the number of atoms in a sample, unless you work in atomic physics or smaller.

    Avogadro's constant tells you how many atoms, molecules or ions (still for starters) there are in one mole of that substance.
  16. Feb 2, 2010 #15
    I finally realize the avogadros and moles but what it asks me to write down examples of moles, what do they expect me to write?

    Is it something like this? "10 moles of Argon gas; (40) + (0) = 40 | 1 mole = 40g x 10 = 400g"

    or is that something completely different?
  17. Feb 2, 2010 #16


    User Avatar

    That is so wrong !?!
    The units are confusing me...

    the correct way would be:

    10 moles of Argon gas
    Atomic weight Ar=40au so,
    M(1 mole Ar)= 40au x 10 moles=400 g

    Na-Avogadro - is a number
    mole - is a quantity
    au- atomic unit (aka ~mass of a hydrogen atom)
    g - gram (metric)
  18. Feb 2, 2010 #17
    Could you please give me another example so I can understand it more easily.


    Maybe Magnesium?
  19. Feb 2, 2010 #18


    User Avatar

    Mg has A=24.2au (that is because there are 3 stable natural isotopes and in different proportins)

    so M(mass)=10molesx24.2au=10*Na*24.2au=10*6*10^23*24.2*1.66*10^-27=~242g

  20. Feb 2, 2010 #19


    User Avatar

    The ideea is that moles are number, a quantity.

    If you multiply a quantity with a mass you get a mass.

    10 moles is not equal with 400 g (for argon). The mass of 10 moles is equal to 400g (Ar).
  21. Feb 2, 2010 #20
    Yes.. heh, which makes my teacher still right.. 10 moles is the same as the mass of 10 moles :\

    But do you know any other examples like mine because the one above is just too confusing and isn't what my teacher expects me to do :P
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook