Chemistry Help

Discussion in 'Community Discussion' started by MonksMac, Apr 13, 2009.

  1. MonksMac macrumors 6502a

    MonksMac

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    #1
    Can someone who really excels at Chemistry help explain Empirical Formula and Molecular formula to me? Please? This stuff is so hard...:(
    Ex. Question-
    "A compound containing only sulfur and nitrogen is 69.6 % sulfur and 30.4 % nitrogen by mass. The molecular mass is 184 g. What is the empirical and molecular formulas of the compound?"
    Can anybody help explain this to me?
    Thank you,
    MM:)
     
  2. Tomorrow macrumors 604

    Tomorrow

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    #2
    I don't excel at chemistry at all, but this one is pretty simple, really. Wikipedia is your friend.

    Simply put: the empirical formula is the ratio of the numbers of atoms (or moles) of each element in a compound. The molecular formula incorporates the actual number of atoms of each element in the compound.

    A good example is glucose. The molecular formula C6H12O6, which is the actual number of each type of atom in the molecule. The empirical formula can be written as CH2O, since there are equal numbers of C and O atoms, and twice as many H atoms (think of it as being similar to simplifying a fraction using least common denominators).

    Another example is hydrogen peroxide. The molecular formula is H2O2, but the empirical formula is HO.

    GIVEN:
    184g of a compound
    69.6% S by mass
    30.4% N by mass

    FIND:
    empirical formula
    molecular formula

    SOLUTION:
    Atomic weight of S = 32.065 ~ 32
    Atomic weight of N = 14.0067 ~ 14

    For S:
    184g * 69.6% = 128.064g S / 32 g/mol = 4.002 mol S

    For N:
    184g * 30.4% = 55.936g N / 14 g/mol = 3.995 mol N

    Since the number of S atoms is approximately equal to the number of N atoms, the empirical formula is SN.

    The most likely molecular formula for this compound is Tetrasulfur Tetranitride, or S4N4 - but it could just as easily be Disulfur Dinitride (S2N2), or any other molecule with an equal number of S and N atoms.
     
  3. MonksMac thread starter macrumors 6502a

    MonksMac

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    #3
    Cool! That's a lot more understandable than what the teacher was trying to tell us how to do it.
    Thank You!
    Edit: Will this also work with Balancing Equations in a similar manner?
     
  4. .Andy macrumors 68030

    .Andy

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    #4
    I disagree. You explained that wonderfully even with the liberal use of wikipedia :).
     
  5. furcalchick macrumors 68020

    furcalchick

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    #5
    it's been a while since i've done this, but to get the empirical formula (that is the basic formula of a compound, for instance, CH2O would be for sugar), multiply 184 by both .696 and .304 (that's 128.064 g of sulfur and 55.936 g of nitrogen). get the molar mass from there (divide the numbers from the last bracket from the atomic weight, it's 32.065 g for sulfur and 14.0067 g for nitrogen) to get the number of mols for both elements.

    there are 4 mols of sulfur and 4 mols of nitrogen. the empirical formula (lowest possible version of the formula is SN and the molecular formula (actual) is S4N4. at least i think so. someone with more recent chem experience should either confirm or deny this answer.
     
  6. MonksMac thread starter macrumors 6502a

    MonksMac

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    #6
    I agree too. I wish you were my teacher!:)
     
  7. furcalchick macrumors 68020

    furcalchick

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    South Florida
    #7
    actually, i only used wikipedia for a couple of numbers, but the majority of my explanation was based off memory, and it wasn't really that hard after thinking about it for a bit. but i think the first explanation was alot clearer than mine was.
     
  8. .Andy macrumors 68030

    .Andy

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    #8
    Yours was good too furcalchick :).
     
  9. MonksMac thread starter macrumors 6502a

    MonksMac

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    #9
    This stuff doesn't seem too hard, but my teacher comes up with the most convoluted ways to explain this stuff- at the Sophmore/Junior-Pre AP level. She's a nice person, but not so good as a teacher. Sometimes I don't think she even knows what she is teaching...:eek:
     
  10. Tomorrow macrumors 604

    Tomorrow

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    #10
    Thanks! I actually did spend some time as a teacher, but I haven't done anything with chemistry since my freshman year of college (way, way back in '87).

    Good luck in your class!
     
  11. .Andy macrumors 68030

    .Andy

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    #11
    I used to think the same way about my science teachers. I used to find it difficult to understand some of their ways in explaining things. That's the problem with a classes - one method of explanation might not suit everyone. In the end with the sciences (like just about every subject) it's best to treat the didactic lessons as an launch point for your own understanding. Find a textbook(s) that you like and make good use of the internet and hassle friends/family/associates for their explanations. As long as you're a bit tenacious and show initiative the sciences are unbelievably rewarding :).
     
  12. MonksMac thread starter macrumors 6502a

    MonksMac

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    #12
    Don't get me wrong- I LOVE science stuff, but not so much chemistry because I don't have the strongest math background (algebra). I find biological sciences much more to my liking, but Bio,Chem, and Phys. are required for HS graduation in TX. and starting with my graduating class we must take an additional year of both science and math. Delightful.;)
     
  13. andy rodick macrumors newbie

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    #13
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