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Deducing a Molecular Formula from Mass Spectra

  1. Oct 28, 2014 #1
    1. The problem statement, all variables and given/known data

    When reacted with a small quantity of water, SiCl4 produces an oxychloride X, SixClyOz. The mass spectrum of X shows peaks at mass numbers 133, 149, 247, 263 and 396. You should assume that the species responsible for all these peaks contain the 16O, 35Cl and 28Si isotopes only.

    2. The attempt at a solution

    The first step is fairly straightforward; the peak responsible for X is the 396 peak. Things start to get a little foggy when finding x, y and z.

    ##28x + 35y + 16z = 396##

    How can I go about solving such an equation without using some sort of time-consuming trial and error method?
     
  2. jcsd
  3. Oct 28, 2014 #2

    Borek

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    Staff: Mentor

    I would start assuming Si is bonded only to oxygen and chlorine (no Si-Si bonds) and there are no O-Cl bonds. That should give another equation, so you will be left with two unknowns only.
     
  4. Oct 28, 2014 #3
    I don't see why there can't be Si-Si bonds.
     
  5. Oct 28, 2014 #4
    Same thing goes for O-Cl bonds.
     
  6. Oct 28, 2014 #5
    ##28x + 35y + 16z = 396##
    ##x = z+1##
    ##y = 2x + 2##

    Therefore:

    ##x = 3##
    ##y = 8##
    ##z = 2##

    Which happens to me the right answer in my answer key. However, Si4Cl4O9 and Si8Cl4O2 are also given as acceptable alternatives.
     
  7. Oct 28, 2014 #6

    Borek

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    Staff: Mentor

    It is not that Si-Si (or O-Cl) bonds are impossible, but in general in silicates silicon atoms are connected by an oxygen "bridge": Si-O-Si. Chlorine can replace oxygen, yielding Si-Cl, but it won't link two Si atoms like oxygen does.

    Not sure what your equations are intended to mean. What I was hinting at is that all silicon valences are being used for bonds with oxygen and silicon, so 4x=y+2z.
     
  8. Oct 29, 2014 #7
    Could you please elaborate on how exactly you arrived at this equation? My reasoning was based on drawing a couple of structures wherein Si atoms are joined by O bridges and noticing the pattern of how the number of Si atoms varies with the number of O atoms (##x = z + 1##) and how the number of Cl atoms varies with the number of Si atoms (##y = 2x + 2##).
     
  9. Oct 29, 2014 #8

    Borek

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    Staff: Mentor

    4x is a number of bonds that x atoms of silicone silicone can make. y+2z is number of bonds that y atoms of chlorine and z atoms of oxygen can make. All are used, and as silicon bonds only to oxygen or chlorine, 4x=y+2z.

    It is just an analogy, you would use the same formula to describe an ionic compound where Si is always +4, Cl is -1 and oxygen is -2. Silicon oxychloride is not an ionic compound, but the analogy works.
     
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