Homework Help! (Chemistry)

Discussion in 'Community Discussion' started by TSE, Oct 27, 2008.

  1. TSE macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
    #1
    I need help with homework! Can you please tell me how you got it?

    Find the number of representative particles in each substance. (Use correct significant figures).

    (a) 2.78 mol Sn
    ________atoms
    (b) 0.590 mol KCl
    ________ formula units
    (c) 7.49 mol SO2
    ________ molecules
    (d) 5.91 multiplied by 10-3 mol NaI
    ________ formula units
     
  2. wvuwhat macrumors 65816

    wvuwhat

    Joined:
    Sep 26, 2007
    #2


    This is precisely why I'm a Communication Studies major...
     
  3. ucfgrad93 macrumors P6

    ucfgrad93

    Joined:
    Aug 17, 2007
    Location:
    Colorado
    #3
    Well, a mole is equal to 6.023X10^23 atoms/molecules/particles.

    So, multiplying that by 2.78 = 1.67X10^24, for the first question.

    Hopefully, you can do the rest. Good luck.
     
  4. TSE thread starter macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
    #4
    Wait so you multiply 2.78 by mol and that's the answer?
     
  5. ucfgrad93 macrumors P6

    ucfgrad93

    Joined:
    Aug 17, 2007
    Location:
    Colorado
    #5
    Yes.

    Let me know what you get on your homework. Hopefully, I didn't lead you astray.;)
     
  6. gonyr macrumors 6502

    Joined:
    Jul 9, 2006
    Location:
    Niagara County, NY
    #6
    If you need to show your work (you probably do):

    given # moles / 1 X 6.02 x 10^23 particles / 1 mole


    BTW, all of your given measurements have three significant digits, so your answer should as well. My class has a test on mole problems tomorrow.
     
  7. furcalchick macrumors 68020

    furcalchick

    Joined:
    Dec 19, 2006
    Location:
    South Florida
    #7
    actually you multiply it by Avogadro 's number (6.023x10(to the 23rd power)). same applies for c. btw.

    for b and d, is formula units the molar mass of the molecule or something like that? if so, you get the mass of the molecule and multiply by the number of mols. correct me if i'm wrong btw.
     
  8. ucfgrad93 macrumors P6

    ucfgrad93

    Joined:
    Aug 17, 2007
    Location:
    Colorado
    #8
    Doesn't B only have 2? Its been awhile since I had to figure this out, so I could easily be wrong.
     
  9. gonyr macrumors 6502

    Joined:
    Jul 9, 2006
    Location:
    Niagara County, NY
    #9
    No, formula units is functionally equivalent to molecules. It's used for ionic compounds that aren't made of individual molecules like covalently bonded compounds. You would only use a formula mass if the question asked for the number of grams.
     
  10. gonyr macrumors 6502

    Joined:
    Jul 9, 2006
    Location:
    Niagara County, NY
    #10
    No, the zero at the end of a number, after the decimal point, is a measured digit. It shows the precision of the measurement. There's a difference in meaning between 0.5 and 0.5000.
     
  11. richard.mac macrumors 603

    richard.mac

    Joined:
    Feb 2, 2007
    Location:
    51.50024, -0.12662
    #11
    atoms = Avagadro's number x mol

    molecules = mol x molecular mass (m=n x M)

    a formula unit is the empirical formula of an ionic or covalent compound and is the lowest whole number ratio of ions

    so in this case as KCl and NaI are ionic and in lowest ratio, molecules = formula units
     
  12. gonyr macrumors 6502

    Joined:
    Jul 9, 2006
    Location:
    Niagara County, NY
    #12
    This is incorrect. The formula you have given will find mass (m), not the number of molecules.

    Also, in my experience, formula unit is only used for ionic compounds, never covalent.
     
  13. ucfgrad93 macrumors P6

    ucfgrad93

    Joined:
    Aug 17, 2007
    Location:
    Colorado
    #13
    Ah, got it, thanks! Like I said it has been awhile, about 15 years.:D
     
  14. Chundles macrumors G4

    Chundles

    Joined:
    Jul 4, 2005
    #14
    Isn't the answer just "Jesus"?

    But seriously folks...

    Get really comfortable with Avagadro's number, you'll have it etched in your brain by the time you're done with Moles. It's a bit difficult to get your head around but it's just another unit of measurement. Just like 6.02X10^23 carbon atoms is a mole of carbon, the same number of oranges is a mole of oranges. But, whereas we'd have no idea of how much a mole of oranges weighs, we're lucky in that one mole of any element's atoms will weigh exactly it's molar mass in grams.
     
  15. furcalchick macrumors 68020

    furcalchick

    Joined:
    Dec 19, 2006
    Location:
    South Florida
    #15
    just double checking. didn't see formula units much.
     
  16. gonyr macrumors 6502

    Joined:
    Jul 9, 2006
    Location:
    Niagara County, NY
    #16
    A good way to think of it is as a counting unit, like a dozen, only much, much bigger. Just like a dozen is 12 of something, regardless of what the something is (like eggs, donuts, etc.), a mole is 6.02 x 10^23 pieces of something, regardless of what the thing is.
     
  17. ChrisA macrumors G4

    Joined:
    Jan 5, 2006
    Location:
    Redondo Beach, California
    #17
    The key here is that a mol or ANYTHING always has the same number of particles. That is the whole point of the "mol". It makes the math really easy. Chemists invented the mol back when no one had calculators. Some smart guy figured out that if you used molls rather then grams you don't always have to divide or multiply buy some ugly number that you really don't know exactly.

    Call the number o particles per mol "A" so then 2 mol has 2A particles and 2.435 mol has 2.435A particles and so on.
     
  18. ChrisA macrumors G4

    Joined:
    Jan 5, 2006
    Location:
    Redondo Beach, California
  19. Iscariot macrumors 68030

    Iscariot

    Joined:
    Aug 16, 2007
    Location:
    Toronteazy
    #19
    Also delicious in salad.
     
  20. richard.mac macrumors 603

    richard.mac

    Joined:
    Feb 2, 2007
    Location:
    51.50024, -0.12662
    #20
    my mistake… whats the formula?

    i got the Forumla Unit definition from Wikipedia
     
  21. Clyde099 macrumors regular

    Joined:
    Sep 30, 2007
    #21
    What do you exactly do with one of those?
     
  22. ucfgrad93 macrumors P6

    ucfgrad93

    Joined:
    Aug 17, 2007
    Location:
    Colorado
    #22
    Become somebody's spokesman.;)
     
  23. TSE thread starter macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
  24. TSE thread starter macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
    #24
    New question!!!!!!

    How can you determine the molar mass of a gaseous compound if you do not know its molecular formula?
     
  25. Chundles macrumors G4

    Chundles

    Joined:
    Jul 4, 2005
    #25
    Something about the Ideal Gas Law rings a bell but I'm not sure. Been years since they bestowed that bloody degree on me.
     

Share This Page