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Homework Help: Partial pressure quest

  1. Nov 12, 2012 #1
    We have an empty vessel with volume of 2L. We put 2.42gr of PCl5 (g) and allowed it to partially decompose at 250 Celsius according to:

    PCl5 --> PCl3 + Cl2

    the two prodcuts are also gases. The total pressure inside the vessel after this partial decomposment is 359 torr. What is the partial pressure of each gas now?

    I calculated that 0.0116 mol of PCl5 were inserted to the vessel, and I choose x to be the amount of molls of PCl5 that became the 2 products, so we have x molls of PCl3, x molls of Cl2 and 0.0116-x molls of PCl5 at the end. But I don't understand - if we add up this amount we get 0.0116+x molls at the end, but we started from 0.0116 molls of material in the first place... I don't understand how this makes sense...
    Last edited: Nov 12, 2012
  2. jcsd
  3. Nov 12, 2012 #2
    somebody? please??
  4. Nov 13, 2012 #3
    If 1 mole of PCl5 gets completely decomposed into the two products, what is the total number of moles you get?
  5. Nov 13, 2012 #4
    2... You got me here also.. Still dont get how this all makes sense
  6. Nov 13, 2012 #5
    Why doesn't it make sense? :confused:
  7. Nov 13, 2012 #6
    Because you start from an amount of 1 moll and get 2 molls... But maybe because these 2 molls are not made up each of exactly what made up the original 1 moll its ok..
  8. Nov 13, 2012 #7


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    Staff: Mentor

    Number of atoms - doesn't change. Total mass - doesn't change. But total number of moles of substances can change any way you like (doesn't mean it always does!). Think water synthesis from elements - 2 moles made out of 3 moles. Think carbon combustion to CO2 in oxygen - 1 mole made of 2 moles. Write reaction equations, balance them and you will see.
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