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Units question (g mol)

  1. Jan 27, 2005 #1
    I am currently taking a Unit Operations course. The book and the instructor both refer to a unit, g mol (gram mol). The book defines molarity as 1 (g mol)/Liter. I am familiar with molarity but always in the past it was defined as mol/Liter. I do not understand what this unit is supposed to be. Mass? mols? Can someone clear this up for me?

    The text I am using is Transport Processes and Separation Process Principles, 4th edition, Christie Geankopolis.

  2. jcsd
  3. Jan 27, 2005 #2


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    "gmol" --- "relative" molecular mass times grams; compare to the kgmol, lbmol, tonmol (English or metric --- gonna be different). The chemists got here first, and a "mole" (mol) is understood to be a g-mol --- had chemists been raised in religious awe of the English units system, we might have a convention in which the unspecified "mol" would be understood to be the dram, scruple, poise, oz., grain, or some other irrational mass unit times molecular mass (weight).
  4. Jan 27, 2005 #3


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    To summarize (at the cost of sounding repetitive) : "g mol" is the same as "mol", as per accepted convention.
  5. Jan 28, 2005 #4


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    "Mol"is in perfect connection with "gram" and "Kilomol" with "Kilogram".Par éxample
    18 grams of water =1 mol
    18 Kilograms of water=1 Kilomol

  6. Jan 28, 2005 #5


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    Even though this isnt exactly related I think it might help you understand


    where C is the molar concentration in moles per liter (mol/L)
    V is the volume in Liters
    n is the number of moles of solute

    I'm doing this type of stuff now, so here's and example:
    1 mole of H2SO4 = 98 g
    What is the volume of a solution whose concentration is 0.2 mol/L if it contains 49 grams of sulphuric acid.

    So in looking for the volume you have

    but n must be in moles not grams so

    [tex]\frac{1 \\ mole}{98 \\ g}=\frac{x \\ mole}{49 \\ g}[/tex]
    Cross multiply and you get 0.5 mole is equal to 49 grams of sulphuric acid. (though its just haf and half :wink: )

    Now when you use the formula you always want your units to cancel out and in this case be left with L.

    [tex]V=\frac{0.5 \\ mol}{\frac{0.2 \\ mol}{L}}[/tex]

    mol cancels and you're left with L.

    [tex]V=\frac{0.5}{0.2}=2.5 Liters[/tex]

    So mole and grams are interchangible just with a little math and sometimes not even, for example 39 grams of Potasium (atomic mass) is equal to 1 mol of atoms, and so on.
    Maybe it helped.
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