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

Fundamental problem regarding mole concept

  1. May 28, 2017 #1
    Background: Amount of substance is a fundamental physical quantity which has mole (mol) as it's SI unit. Therefore all expressions for amount of substance should have the unit mole on simplification.

    Therefore, if A = Amount of substance (normally in moles), m = Mass of the substance in a particular unit (normally in grams), M is the mass per unit amount of substance (normally in grams/mole), then,

    A = m/M

    If the substance is a molecule then M is known as molar mass of the substance.

    But in my textbook and in many websites on the internet, I have encountered the phrase number of moles. I think it refers to the amount of substance or may be the numerical part in the amount of substance. The formulas given there were strange.

    If n is the number of moles, m is the mass of the substance in grams and x is the atomic weight (for atoms of elements) or molecular weight (for molecules of elements and compounds), then,

    n = m/x

    This equation is not dimensionally correct if I am right.

    My questions:

    First of all i would like to ask whether amount of substance and number of moles refer to the same thing or is it that amount of substance has a unit along with a numerical value whereas number of moles does not have a unit and represents the numerical value in the magnitude of the amount of substance.

    Secondly, between equations 1 and 2, which one is completely correct (both in meaning and dimension)?

    Thirdly, since in some place I have encountered gram atomic/molecular mass in place of molar mass, i would like to know what are the differences between both and do they have the same units or different units?
  2. jcsd
  3. May 28, 2017 #2
    The only difference is that in first one you have ##g/\mathrm{mol}## as the unit of denominator whereas it is ##g## in second one. First one is dimensionally correct but it does not matter.

    Amount of substance can be in dozens but number of moles will be in moles.
  4. May 28, 2017 #3


    User Avatar
    Science Advisor
    2017 Award

    mol is just one unit for the amount of a substance and you could use other units as well (a mol of atoms, a dozen eggs, a ream of paper). Amount of substance can be converted to number of moles through Avogadro's constant: 6.02e23/mol.

    For example, for one helium atom * mol/6.02e23 = 1.66e-24 mol. Similarly, one helium atom * dozen/12 = 0.083 dozen.

    Atomic/molecular weights are generally given in g/mol so taking a mass and dividing it by the molecular weight will give number of moles.
  5. May 29, 2017 #4
    @Buffu , so number of moles has the unit moles. Right?
  6. May 29, 2017 #5


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    MOLES is used as a unit of measure, and can be abbreviated "mol". This unit is the count of 6.022*10^23 of whatever pieces are being counted.

    DOZEN is used as a unit of measure, abbreviated in whatever way will be useful or understood. This unit is a count of 12 of whatever pieces are being counted.
  7. May 30, 2017 #6


    User Avatar
    Science Advisor

    Yes, therefore, I think that amount of substance should be the preferred word, which is also the view upheld by IUPAC.
  8. May 30, 2017 #7

    stefan r

    User Avatar
    Gold Member

    Moles are a better unit for making some statements. For example, "2 moles of hydrogen and 1 mole of oxygen combine to form 1 mole of water". That is close to "2 grams of hydrogen and 16 grams oxygen make 18 grams water" but not exactly. The hydrogen and oxygen could have isotopes so the mass can vary depending on how much deuterium, oxygen18 we mix in. Pure single proton hydrogen also has slightly more mass than 1 gram per mole. Then there is a slight loss in mass because energy is released in the chemical reaction. If you state the equation using mols you can disregard the extras and just express the chemical reaction.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted