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Homework Help: Molar mass of each substance

  1. Oct 7, 2008 #1
    How many moles of each substance are contained in 88g of carbon dioxide?
    Since it's CO2:
    Ar of carbon= 12
    Ar of oxgen = 16
    Mr of Carbon dioxide = 44
    88g/44= 2 Mol of CO2 formula units
    Since the ratio is 1:2 carbon to oxygen:
    (88/3)/12=22/9 Mol of Carbon
    (i.e. 1/3 of 88g is how many grams carbon represents?)
    (176/3)/16=11/3 Mol of oxygen
    (i.e. 2/3 of 88g is how many grams oxygen represents)
    Is this right? Any replies much appreciated! Thanks in advance!
  2. jcsd
  3. Oct 7, 2008 #2
    This part is good, but afterward I'm not quite sure how your calculations go. Here's a simpler way, I believe:

    You have 2mol of CO2. This means you have 2mol C and 4mol O (each atom is multiplied by the mole amount (2).)
    2mol C = 2mol(12g/mol) = 24g
    4mol O = 4mol(16g/mol) = 64g
    Make sure all masses add up to the total mass (88g), and that's it! It gets a bit more complicated for larger molecules (e.g. H3PO4) or with messier numbers (8.57mol rather than 2mol) but they should work the same way.

    So this is the mass of each; if you wanted percent by mass (common for high school or basic college chem):
    C = 24g/88g x 100 = ~27%
    O = 64g/88g x 100 = ~73%
    Last edited: Oct 7, 2008
  4. Oct 7, 2008 #3
    I don't know where they were going either.
    Thanks for the help though!
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