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Hard problem

  1. Nov 29, 2004 #1
    My teacher is ridiculous. We didn't learn anything about boiling points or Van Hoft factors or whatever. The problem are in the attachments.

    From the description i can only predict the equation as

    S8 + AgF -> SF2 + Ag

    I'm not even sure if it is right or not

    I have trouble on the second equation.

    As well using pv= nRT,

    20 kpa * (mass of SF2)/.083 g/L = n*8.314*305

    If i have the mass of the Gas i might be able to figure out the moles of the
    I can't even figure out the second equation so even with the given information i cannot proceed with this problem.

    In fact i'm not even sure if i'm starting the problem right or not.

    I don't even know the transformed structure of the gas either....

    Please help. I really need this. thanks in advance.

    Attached Files:

  2. jcsd
  3. Nov 30, 2004 #2
    Hmm, these problems seemed to be a bit cut off so if you could just copy the text it might help us out a bit.

    OK, well first of all you are not sure of the exact form of the product. It could be SF2, but it could also be something else like SF3, SF4 .... The point of the problem is to determine the molecular formula of what is formed.

    You use the ideal gas equation, which is correct. You are trying to find the molecular weight, but molecular weight doesn't appear in the ideal gas equation. You are given the density, lets call it d. How do the density and molecular weight(call it MW or something appropriate) relate to n and V? In other word s you will end up with something like f(n, V) = g(d, MW) Can we rearrange the ideal gas equation to replace n and V with d and MW, thus reducing our unknowns to one and allowing us to solve the problem?

    Also, be careful with your units. 8.314 what? 305 what ? If the units don't cancel properly, you know you have the wrong answer.
  4. Nov 30, 2004 #3


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    I read the first cut off. It reads that the dry gas (whatever it is) reacts with water to give hydrogen fluoride, HF.

    A second portion of the question involves calibrated amounts of dry gas and water to yield a known amount of hydrogen fluoride.

    So let me write the possible reaction like that:

    [tex]S_8+32AgF\longrightarrow 8SF_4+32Ag[/tex]

    The sulfur tetrafluoride is the "dry gas" in this question, I suppose. It might give hydrogen fluoride with the action of water like this:

    [tex]SF_4+2H_2O\longrightarrow SO_2+4HF[/tex]

    So, consider these reactions to check if they give the correct results.
  5. Dec 1, 2004 #4


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    From the first portion of the information provided, you can find the molar mass of the initial gas formed. Use pv=nrt,

    from density, grams/L, transform to moles/L.......simply put the equation in the form grams X molar mass/L and, or (.803 +/- .010)x/L, where x is the molar mass

    P/(.803 +/- .010)RT = x, solve for x (if you wish to keep the plus/minus estimate, you'll need to work it out through the whole problem which will be a bit more tedious.

    That should get you started, I don't have time to work through the whole problem at this point, in fact you might want to double check the answer and concept with your teacher.
  6. Dec 1, 2004 #5
    It sure would help to have the full problem posted! Anyway, it says that the initial product boils at 15 deg.C. That's useful info! Also, a question: what level of chemistry class is this for? What's it called? That may influence what sort of equation-of-state (gas law) your teacher/professor expects you to use.
  7. Dec 1, 2004 #6
    Oooops! I missed it; you said "teacher"...if this is for high-school chemistry, then the ideal gas law is OK.
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