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Avogadro’s number type of question help.

Discussion in 'DAT Discussions' started by Smooth Operater, Aug 1, 2006.

  1. Smooth Operater

    Smooth Operater don't bug "operatEr"!
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    Avogadro’s number is NOT equal to:
    A . the number of atoms in 11.2 L of O2 at STP.
    B . the number of atoms in 1 mole of He at STP.
    C . the number of electrons in 96,500 coulombs.
    D . the number of ions in 1 L of 1 N sulfuric acid.

    the correct answer is D.

    But, I don't understand the reason stated on my answer key. Can anyone elaborate on the reason behind choosing D. Thanks !
     
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  3. allstardentist

    allstardentist All-Star
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    first of all, u can get D by process of elimination because we know that A,B,C are correct for sure. What was their explanation for it?
     
  4. dat_student

    dat_student Junior Member
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    N = Normality

    Normality is expressed in equivalents per liter. An equivalent can be defined as the number of moles of "reactive units" in a compound.

    Now let's look at sulfuric acid. Each molecule of sulfuric acid can dissociate to yield two hydrogen ions and a sulfate ion in solution, so one mole of sulfuric acid can form two moles, or two equivalents, of hydrogen ions. For sulfuric acid, 1 M (1 mole per liter) will have two moles of hydrogen ions per liter, so it will be a 2N solution (2 equivalents of hydrogen ions per liter). Thus, 1N = 0.5 M which is not equal to Avogadro's number
     
  5. allstardentist

    allstardentist All-Star
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    but u know DAT student, in the kaplan book, it says that normality depends on the type of reaction. In the case of acid-base reaction, your reasoning would be correct, but in the case of sulfate precipitation, it is stated that 1 normal would yield 1 mole of sulfate ions. Seems like all the questions we post on sdn are posted because they usually dont seem to have a clear cut answer.
     
  6. keibee82

    keibee82 Blue_tooth...
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    yes, part D is referring to sulfate ions, not protons. I don't see why
    D is wrong. 1N in 1L means 1M.
     
  7. Yellow Snow

    Yellow Snow Senior Member
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    It says Ions which includes the H+ and the sulfate ions, no? So wouldn't we have two moles of H+ and one mole of sulfate thus 3 x avo's number?
     
  8. Indie

    Indie Member
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    Sorry read the question wrong...
     
  9. keibee82

    keibee82 Blue_tooth...
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    This question is from kaplan practice exam 4.
    For part E on the original, it says...
    The number of SO4 ions in 1 liter of 1 N H2SO4.

    so they are referring to SO4 ions which is 1 N, meaning 1 M.
     
  10. lifeisgood

    lifeisgood the original
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    It's simply a tricky question...I think it is safe to assume when talking about acids, unless you are talking in the context of a certain rxn like a sulfate ppt, that normality refers to the H+ concentration. SO, since one liter of a 1N solution will contain one mole of H+, the sol'n is 1M wrt H+...likewise since there are twice as many H+'s as SO42-, the sol'n is only 0.5M wrt to SO42-. One half a mole of SO42- in one liter is only equal to half of Avogadros number. Hope this helps.
     
  11. DonExodus

    DonExodus Dentist in Virgin Islands
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    For every mole of H2SO4, there are 2 H+ ions released.
    Hence, there is 1 ion released per 1/2 mole (simplified of course), 2 released per 1 mole.

    Since there is 1/2 a mole, its 1/2 avagadros number, making D correct.
     
  12. dat_student

    dat_student Junior Member
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    Exactly. You guys explained it better :)
     
  13. allstardentist

    allstardentist All-Star
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    do we just assume its a acid-base rxn?
     

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