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Mass % of an alloy

  1. Oct 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A 0.250-g sample of a magnesium-aluminum alloy dissolves completely in an excess of HCl(aq). When the liberated H2(g) is collected over water at 29C and 752 torr, the volume is found to be 313 mL. The vapor pressure of water at 29C is 30.0 torr.

    2. Relevant equations

    PV = nRT
    Al = 26.98 g/mol
    Mg = 24.31 g/mol
    3. The attempt at a solution

    I'm unsure how to do this...but here's what I did.

    1) First I converted everything.

    752 torr = 0.989 atm
    313 mL = 0.313 L
    29C = 302K

    2) Then I used the forumla PV = nRT to find out how many moles of water there was.

    3) Then I multiplied the moles by the MM of Al and Mg.

    4) Finally I did .3038x + .3371(1-x) = 250 and got a whole number above 100% which means that my answer was wrong.

    I haven't used any the information about the vapor pressure of water because I dont know what to do with it...
  2. jcsd
  3. Oct 17, 2010 #2


    User Avatar

    Staff: Mentor

    Start with the reaction equations. Water doesn't tell you anything about alloy composition.

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