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Homework Help: Chemistry help

  1. Sep 8, 2005 #1
    [urgent] chemistry help

    I am given the percent by mass of various elements in an unknown compound, I must then find the empirical formula then find the molecular formula by the molar mass of the compound in part b. This particular problem, however, does not have the mass percentages add up to 100%, more like 68%, so I have to guess what the remaining 32%, and my assumptions vastly affect the outcomes of my asnwers.

    Question: The percentages bt nass of C, H, and N in an unknown compound are found to be 23.30%, 4.85% and 40.78%, respectivley. (N.B. these do not add up yo 100%. Why?).
    (a)Determine the empirical formula of the compound.
    (b)The molar mass is 206 g/mol, what is its molecular formula?

    Solution attempt:

    The only think that could account for as much as 32% of the compound's mass that I know of is [tex]H_2 0[/tex] if it is hydrated. So therefore the other 31.07% of the mass is oxygen. So a little bit of work shows that the mole ratio is as follows : [tex] C : H : N : O = 1 : 2.47 : 1.5 : 1 [/tex]
    so I multiply everything by two to get numbers that can be rounded to whole numbers
    [tex]2 : 5 : 3 : 2 [/tex]
    Since the Oxygen is part of the water, 2 moles o oxygen requires two 4 moles of hydrogen, so the resulting empirical formula is [tex]H C_2 N_3 - 2 H_2 O [/tex]

    some more calculations show that the molecular formula is ROUGHLY that times 2 for.
    That compound has no polyatomic ions in my table. Am I correct?
  2. jcsd
  3. Sep 8, 2005 #2
    would I get a faster response if I posted this in the chemistry forum? Is cross-posting allowed?
  4. Sep 8, 2005 #3
    Do you mean they are given to you in percents, as in grams? If this is the case, convert to moles and divide each by the smallest molar mass. Then you should be able to set up your ratio.
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