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Avogadro's Law

  1. Feb 3, 2015 #1
    Avogadro's Law states that 'equal volumes of gases at the same temperature and pressure contain the same number of molecules' or moles of gas.
    My question is ,will it be correct if we reverse the law,
    I mean if gases of equal moles at the same temperature and pressure will have equal volumes,is it right?For eg.
    • N2(g) + 3H2(g) ==> 2NH3(g)

    • 1 mole of nitrogen gas combines with 3 moles of hydrogen gas to form 2 moles of a ammonia gas.we can say that 1 volume (what ever be the unit) of nitrogen reacts with 3 volumes of hydrogen to produce 2 volumes of ammonia?
    • provided that the gases are at same temperature and pressure.
     
  2. jcsd
  3. Feb 3, 2015 #2

    Borek

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    Staff: Mentor

    Yes, it works both ways.

    (As long as the ideal approximation holds)
     
  4. Feb 3, 2015 #3
    Are you talking about this point -
    • provided that the gases are at same temperature and pressure.
     
  5. Feb 3, 2015 #4

    Borek

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    No. PV=nRT is an "ideal gas approximation" - that is, it works only for ideal gases. No gas is ideal, but as long as the distances between molecules are large enough, every gas can be approximated as ideal.

    http://en.wikipedia.org/wiki/Ideal_gas (just read the initial part).
     
  6. Feb 3, 2015 #5
    I have one more problem
    • Given the equation: 2H2(g) + O2(g) ==> 2H2O(l)
      If 40 dm3 of hydrogen, (at 25oC and 1 atm pressure) were burned completely ...What mass of water is formed?
    • In my textbook solution is as follows
      • The easiest way to solve this problem is to think of the water as being formed as a gas-vapour.

      • The theoretical gas volume ratio of reactant hydrogen to product water is 1 : 1

      • Therefore, prior to condensation at room temperature and pressure, 40 dm3 of water vapour is formed.

      • 1 mole of gas occupies 24 dm3, and the relative molar mass of water is 18 g/mol
        • (atomic masses H = 1, O = 16, so Mr(H2O) = 1 + 1 + 16 = 18).
      • Therefore moles of water formed = 40/24 = 1.666 moles

      • Since moles = mass / formula mass

      • mass = moles x formula mass

      • mass water formed = 1.666 x 18 = 30g of H2O
        My question is how can we think water in liquid as water in vapor phase?How is it correct to do?
     
  7. Feb 3, 2015 #6

    Borek

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    It is a shortcut - if water were a gas, vapor would occupy 40 L. Yes, it condenses next, but assuming it is still a gas after the reaction simplifies calculations. You can think in terms of "when gases react, product is gaseous, and the condensation is not instantaneous, so the product is gaseous for long enough we can calculate its amount" ;)
     
  8. Feb 3, 2015 #7

    epenguin

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    Yes, in fact empirical observations of just such combining volumes, subsumed in 'Gay-Lussac's Law' were most of the motivation of the Avogadro hypothesis as it was originally.
     
  9. Feb 3, 2015 #8
    I wanted to ask will mass of water be same in both vapor and liquid phase?If yes,then we can calculate mass of water in any of these phases depending upon ease.
     
  10. Feb 4, 2015 #9

    Borek

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    Staff: Mentor

    Yes, mass is conserved.
     
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