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Homework Help: When hydrogen and oxygen gas combine to form water vapor

  1. May 10, 2005 #1
    Just took a chemistry test, had a few I wasn't sure of:

    When hydrogen and oxygen gas combine to form water vapor, what mass of water forms if 10grams of oxygen reacts completely?

    [tex] 2H_2 + O_2 \rightarrow 2H_2O [/tex]

    10 grams of oxygen is .625 moles, and the stoichiometric ratio is 1:2, so 1.25 moles of water form. 1.25 moles of water has mass 22.5g, but that wasn't one of the options.

    If one mole of Cl2, SO3, H2, and Br2 are placed in the same container, what is NOT the same for each?

    a) Average KE
    b) Average velocity
    c) Gas molecules
    d) Vapor (or partial) pressures

    I put D, because a & b are analogous to each other, and rely on temperature. I almost put c, I caught myself by remembering that a mole of a substance has the same number of gas molecules, but my thinking was for example SO3 has twice as many atoms as the others.

    A face centered cubic holds how many atoms in a unit cell?

    One from the corners and the 6 halves on the sides makes 4, right?

    Which of the following has the lowest freezing point?

    1 m (molality) AlCl3
    1 m NaCl
    1 m NaPO4
    couple others

    I know that a solution's FP and BP change when dissolved, so I figured the one with the most dissolved molecules would have the lowest freezing point. I think I put AgCl3.

    Given the graph of four solutions (Vapor pressure vs temperature), which has the weakest intermolecular forces?

    Solution A had a pretty steep exponential slope, solutions B & C were inbetween, and solution D had a very very shallow exponential slope. I said A, because strong intermolecular forces means its hard to break the bonds in the liquid to form vapor, so the inverse is true also.

    Last edited: May 10, 2005
  2. jcsd
  3. May 10, 2005 #2


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    Take another look at that.

    Same average kinetic energy does not imply same average velocity unless you are talking about particles of equal mass.

    That looks right. I don't know off-hand about the rest.
  4. May 10, 2005 #3
    Im staring hard at [itex] 2H_2 + O_2 \rightarrow 2H_2O [/tex] and I honestly cant see why it would be any different. Wait, is it because its [itex] O_2 [/tex] and not O? Is it 1.25 moles of oxygen?

    Was the answer to the second one correct?
  5. May 10, 2005 #4
    KE IS temperature, so you were right on that.

    velocity however is dependant on mass.

    c as you guess isn't correct either becuase moles and molecules have a 1 to 1 correspondance.

    Partial pressures only depend on the number of moles of the gas, and since they are all equal, their partial pressures should be equal.

    the correct answer in this case is B

    Also, on the first question, Oxygen (the atom) has a molar mass of 16, but Oxygen (the gas) has a molar mass of 32.
    Last edited: May 10, 2005
  6. May 10, 2005 #5


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    What is the molar mass (Mol Wt) of O2 ?

    D is not correct. (what is the expression for partial pressure in terms of mole fraction ?) See Olderdan's and cair0's posts on the correct answer.

    This is the correct answer ! Did you get it wrong ?

    Why would 1 molal AlCl3 have more dissolved "molecules " ? Recall the role of the van't Hoff factor in colligative properties.

    Within the scope of this problem, I think this is okay. I would pick A too.
  7. May 10, 2005 #6
    I appreciate your help. However, the course is introductory, and I really couldnt tell you what van't Hoff factors are. All i know about that problem is molality is mols per kg solvent, and that a solution has a lower freezing point than its solvent alone. What is the right answer?

    A mol of oxygen gas weighs 32g, so 10g is 0.3125mol. 0.625 mol of water will be prodcued which is equal to 11.25g. This is the answer I put, since I figured I made an error somewhere and it probably would have involved a factor of two. :D

    What would you answer for the molality question?
  8. May 10, 2005 #7


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    The van't hoff factor(i) is a pretty basic concept for dealing with colligative properties (Google both terms). It is essentially the number of "particles" per molecule that exist in solution.

    When you dissolve NaCl in water, it breaks up into Na+ and Cl-. So each molecule of NaCl gives you 2 particles. So, the v-h factor, i(NaCl) = 2. The depression of freezing point is proportional to the product of "i" and "m". Since the "m"s are equal, only the "i"s should matter.
  9. May 10, 2005 #8
    Ohh right, something = i K m or something close to that? Since m is the same for all, its just the one with the most number of atoms. NaPO4?
  10. May 10, 2005 #9


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    Umm...not really atoms, but ions. What you've called NaPO4 is really Na3PO4. How many ions does this make ? And the other choices ?
  11. May 11, 2005 #10
    That makes 4, and AlCl3 has 4 also, so it was probably one of the answers I forgot.

    I understand it now, thanks.
  12. May 11, 2005 #11
    Another one I jsut remembered:

    The reaction of sulfur and oxygen to form sulfate follows the equation [itex] 2S + 3O_2 \rightarrow 2SO_3 [/tex], if 3 moles of sulfur and 6 moles of oxygen are supplied, how much sulfate is produced?

    The answers were 2,3,4,5,6 mols. I said three, the limiting reactant is sulfur since 6 moles of oxygen can react with 4 moles of sulfur. Since 3 moles of sulfur are reacting, 4.5 mols of oxygen react, and the ratio between sulfur and sulfate is always maintained. 3 mols of sulfur gives 3 moles of sulfate.

    I know these are silly but I have a tendency to think stupidly and justify incorrect trains of thought.
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