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Chem II - Solubility & Boiling Pts.

  1. Jul 21, 2007 #1

    I needed some help with my Chemistry Homework. I have 4 questions that I'm not so sure of the answer. I've given my best try. Please if someone can help me correct my answers or tell me which one is the right one and how to determine/solve it... Thanks in advance.

    Question One:
    Which gas is expected to be more soluble in H2O?

    a) O2
    b) Cl2
    c) HF
    d) H2
    e) HCl

    ** I think that Oxygen gas is more soluble in H2O.

    Question Two:
    Which produces the greatest number of Ions when one mole dissolves in water?

    a) NaCl
    b) NH4NO3
    c) NH4Cl
    d) Na2SO4
    e) Sucrose

    ** I think its (e) Sucrose. I tried converting the molar mass which (e) has the highest (342.29g/mol) to # of Ions (6.022x10^23)

    Question Three:
    A 1.34m (Molality) aqueous solution of compound X had a boiling point of 101.4 degrees Celcuis. Which one of the following could be compound X? The boiling point elevation constant Kbp for water is 0.52 degrees Celcuis/m (Molality).

    a) CH3CH2OH
    b) C6H12O6
    c) Na3PO4
    d) KCl
    e) CaCl2

    ** I have no clue how to do this one. I tried using the Boiling Point of Elevation equation (delta)(T) = Kbp x m but I don't know what to do afterwards. Please explain this one.

    Question Four:
    The activation energy of a reaction is 76.7KJ/mol. How many times faster will the reaction occur at 50 degrees Celcuis that at Zero (0) degrees Celcuis (assume equal initial concentrations).

    a) 50
    b) 70
    c) 120
    d) 190
    e) 360

    ** Dont know how to approach this question either. Please explain.

  2. jcsd
  3. Jul 22, 2007 #2
    are these questions from your book? what text are you using?

    q1 - which gases have similar intermolecular forces of H2O? the magnitude and strengths of the solute-solute and solvent-solvent bonds should be similar = solute-solvent.

    q2 - think "electrolytes," which ionic compounds will completely dissociate, also which ones are molecular? which one dissociates more, ionic or molecular? also, you don't need to know the molar mass. just knowing whether it's an electrolyte or nonelectrolyte & ionic compound or molecular compound is good enough.

    q3 & q4 - copied word for word? no added information by you?
    Last edited: Jul 22, 2007
  4. Jul 22, 2007 #3
    Hey thanks for your quick help.
    No actually these arent from chem book. The teacher either writes them up or gets from other websites...not sure.

    So for question one: since H2O is a hydrogen bond force, letter (c) HF would be more soluble, since that is also a hydrogen bond ?

    q2: I didn't know about electrolytes, but now I looked it up. And it says that salt is most dissociate-able ? Answer would be letter (a) NaCl
  5. Jul 22, 2007 #4
    No. I havent added/omitted any info. All questions are word to word. is something missing in last two questions?
  6. Jul 22, 2007 #5

    a) O2 -> London Dispersion Forces
    b) Cl2 -> London Dispersion Forces
    c) HF -> Ion-dipole Forces
    d) H2 -> London Dispersion Forces
    e) HCl -> Ion-dipole Forces

    HF & HCl are really close, but I would choose HF. I choose it from the basis that F is more electronegative and smaller than Cl.


    a) NaCl -> 1 Na(+) + 1 Cl(-)
    b) NH4NO3 -> 1 NH4(+) + 1 NO3(-)
    c) NH4Cl -> 1 NH4(+) + 1 Cl(-)
    d) Na2SO4 -> 2 Na(+) + 1 SO4(2-)
    e) Sucrose -> Molecular

    q3, I feel like I'm given way too much information and not enough of what I need. I would think that I would given the mass of the substance rather than the Molality because I can easily find the Molality since I have (delta)T and Kb constant.
    Last edited: Jul 22, 2007
  7. Jul 22, 2007 #6
    (c) would also be considered Hydrogen Bonds since it has F attached to the H. Hydrogen Bonds include N, O, F - Not sure why they are Ion-dipole forces but I'll look it up.

    q2: so then it would be (d) since it has more Ions. ?

  8. Jul 22, 2007 #7
    q1 what is the strongest type of intermolecular force that HF & HCl & H2O have in common?

    q2 yes
  9. Jul 22, 2007 #8


    User Avatar
    Staff Emeritus
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    All questions are correct.

    q1: Think about polarity.
    q2,q3: van't Hoff factor
    q4: Arrhenius equation
  10. Jul 22, 2007 #9
    q3, ah yes i forgot all about the infamous i factor. woohooo! i should sleep earlier.
  11. Jul 22, 2007 #10
    Hi Gokul,

    Thanks for your help. I got q1 & q2....but im still lost with q3 because even knowing the i factor and equation, i dont know how to determine which compound it is....
  12. Jul 22, 2007 #11
    Using the equation calculate the i factor of the compound X, now check which of the compounds from the given choices have the same i factor.
  13. Jul 22, 2007 #12

    I used the equation (delta)Tf = i(Kbm) and solved for i which I got as 1.021.... How do I calculate the i-factors for the compounds when I have no other information? Would I be looking at the number of Ions or moles ???
  14. Jul 22, 2007 #13
    look for what the van't Hoff i factor takes into account.

    i just calculated for the i factor and that's not what i got

    1.4 / (.512 x 1.34) =
    Last edited: Jul 22, 2007
  15. Jul 22, 2007 #14
    ok so when i do that division i get 2.04 =2 Ions ? but where do u get 1.4 from?
    and still what does that division 2.04 result mean? im guessing the answer should be (D) KCl since it has 2 elemnts/ions?
  16. Jul 22, 2007 #15
    (delta)T is 1.4 correct? your Temp was 101.4, BP of H2O is 100.

    i just plugged it in, that's all. yes D is correct.
  17. Jul 22, 2007 #16
    oh ok. i understand it now. We need the final T so we have to subtract 100 from 101.4. and the use the equation and solve for the Ions which is 2 (KCl)

    Thanks for your help with this. It's been very confusing with gases and these equations. I'm glad someone here was to help me.

    I also have 2 another questions which im unsure of. Again, any help will be appreciated.

    Question Five
    What is the mole fraction of acetone, CH3COCH3, in a 45% (w/w) aqueous acetone solution?
    a ) 0.200
    b ) 0.180
    c ) 0.500
    d ) 0.740
    e) None of the above. It is _______

    ** Not sure how to do this one as im confused on converting from mass percent to mole fraction.

    Question Six
    A solution was prepared from equal moles of liquid A (vapor pressure = 15.3 torr), liquid B (vapor pressure = 65.3 torr), liquid C (vapor pressure = 2.39), and liquid D (vapor pressure = 78.3 torr). The vapor of which liquid will have the highest partial pressure above the solution?

    a ) A
    b ) B
    c ) C
    d ) D
    e ) The partial pressure will all be the same.
  18. Jul 22, 2007 #17
    q5 - Assume we have 100g of a solution. They're implying that 45% of it is Acetone (the solute), so that means the rest is?

    q6 - i have no idea how to approach this. i would guess D :-]

    btw, there is a more accurate way to calculate the i factor: but it's not that far off from what you would get by the simpler method.

    i = measured value for electrolyte solution / expected value for nonelectrolyte solution

    which is basically

    i = (delta)Tb of electrolyte solution / (delta)Tb of nonelectrolyte solution
    Last edited: Jul 22, 2007
  19. Jul 22, 2007 #18
    For q6
    Raoult's law states that the vapor pressure due to a volatile component of the system (PA) is

    PA = XAPA°

    where XA is the mole fraction of A and XA° is the vapor pressure of pure A.
  20. Jul 23, 2007 #19


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    Fixing formatting and a typo in the above post:

    Raoult's Law:
    [tex]P_A=\chi_{_A}P_A^o [/tex]

    [itex]\chi_{_A}[/itex] is the mole fraction of A = moles of A / total number of moles
    and [itex]P_A^o[/itex] is the vapor pressure of pure A.
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