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Homework Help: Determination of % Comp of Al/Zn Alloy

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    The purpose of the lab is to determine the % comp of an alloy from the volume of hydrogen that is produced in the reaction.

    We did a lab, in which an alloy was reacted with hydrochloric acid, in a test tube with a stopper and tube leading to a 500ml sidearm flask, with a clamp on the tubing. The sidearm flask was filled with water, and stoppered with a tube going into a 400ml beaker.

    Assuming that the experiment was done correctly, the water from the flask was pushed into the beaker by the exothermic reaction.

    DAta:
    Pressure = 762 mmhg
    Temp = 21.4C
    Mass Alloy Trial One = .1229g, Trial Two = .1251g
    Volume water produced T1 = 131.5ml, T2 = 140 ml.

    2. Relevant equations
    3. The attempt at a solution
    I don't know where to start. =/

    I calculated the amount of hydrogen produced, by calculating percent H in H20, and I got
    14.61ml H2 gas for Trial 1, and 15.56ml H2 gas for Trial 2

    I don't know where to go from here though.. any help would be appreciated
     
  2. jcsd
  3. Oct 14, 2008 #2

    GCT

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    Homework Helper

    Write the relevant reaction equations displaying the products and reactants.
     
  4. Oct 14, 2008 #3
    Zn + HCl -> ZnCl2 + H2
    Al + HCl -> AlCl3 + H2
    if all I can think of..?
    But I don't know how to calculate the amount of hydrogen produced. I know the hydrochloric must be the limiting reagent, to ensure complete reaction, but further than that, I'm stumped.
     
  5. Oct 15, 2008 #4

    Borek

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    Staff: Mentor

    To be sure that reaction proceeded to the end, hydrochloric acid has to be in excess, sot it can't be a limiting reagent.

    If I understand your description correctly, water was not pushed by 'exothermic' reaction, but by evolving gas. Volume of water displaced equals volume of produced hydrogen.

    PV=nRT should give you number of moles of hydrogen.

    Let's take a look at the first trial. Assume there were xg of Al in your sample. If so, there were (.1229g-xg) of Zn. Can you devise a formula that will let you calculate amount of hydrogen produced as a function of x?
     
  6. Oct 15, 2008 #5
    Ummm...

    Would (.1229-x gZn)(1mol/65.4gZn)(1/1) = 4.54
    (x gAl)(1mol/27.0gAl)(3/2) = 4.54

    be the right equations?
     
  7. Oct 15, 2008 #6

    Borek

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    Staff: Mentor

    No idea what 4.54 stands for. And note, that you are interested in TOTAL amount of hydrogen, from both reactions.
     
  8. Oct 15, 2008 #7
    Ah.. the five-day weekend has completely crippled my brain :(

    I think I meant .00545, which is the moles of hydrogen produced from PV=nRT (762mmhg)(.1315L)/(62.4mmhg*L/moles*K)(294.4K).

    I obtained .1315L as the amount of H2 produced, since volume water displaced = volume h2 produced.

    Er. So if I let x = gAl, and .1229-x = gZn, then ((.1229-x)/MZn)(stoichiometric ratio) + (x)(stoichiometric ratio) = .00545 moles H2 produced, and solve for x?
     
  9. Oct 15, 2008 #8

    Borek

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    Staff: Mentor

    Apart from faulty details and unchecked numbers that's the correct idea.
     
  10. Oct 15, 2008 #9
    Um which details are faulty?
     
  11. Oct 15, 2008 #10

    Borek

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    Staff: Mentor

    But you did it OK in the other half of the equation, so thats probably just overlooked.
     
  12. Oct 15, 2008 #11
    Oh! Whoops my bad!
    Thanks so much for your help, by the way!!
    Much Appreciated :D
     
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