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Homework Help: Zinc metal is added to hydrochloric acid to generate......

  1. Nov 18, 2015 #1
    • Please don't ignore the template.
    1. The problem statement, all variables and given/known data
    Zinc metal is added to hydrochloric acid to generate hydrogen gas and is collected over a liquid whose vapor pressure is the same as that of pure water at 20.0°C (18 torr). The volume of the mixture is 1.7 L, and its total pressure is 0.987 atm. Determine the number of moles of hydrogen gas present in the sample.

    0.272 mol

    0.04 mol

    0.997 mol

    0.139 mol

    0.0681 mol

    So theres some container with a volume of 1.7L and pressure of 0.987.... The amount of hydrogen and the other substance in the container are unknown. We also know that the other substance has the same pressure as water does at 20C, (i don't think we use the temperature to find the solution)... And we know the pressure of the other substance is 18torr.

    Note; i'm not sure if the question is saying the pressure of water/the substance at 20C is 18torr. Or if the temperature of the substance is the same as water when the substance is 20C.
    Last edited: Nov 18, 2015
  2. jcsd
  3. Nov 18, 2015 #2


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    Staff: Mentor

    Assume total pressure is the sum of partial pressure of the liquid and hydrogen.
  4. Nov 18, 2015 #3
    So convert 18 torr to atm = 0.0236 atm.
    P total = P1 + P2....
    0.987atm = 0.0236atm + P2....

    So now i know the pressure of the hydrogen. The unknowns are the volume of the other substance and the hydrogen. Thanks for the help with this step!
  5. Nov 18, 2015 #4
    Ok so now its PV=nRT and the answer is 0.0681 mol
  6. Nov 18, 2015 #5


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    Homework Helper
    Gold Member

    I got it, you can get it, fairly fast thinking:

    that's essentially atmospheric pressure and one of the several physical numbers it is useful to remember (was dinned into us at school) is 1 mole gas is 22.4 L (at stp). Water vapour pressure? - negligible by comparison. Thinks of inverted full test tube, you leave it for a time, get a quite small bubble at top, showing this quite small. (Right thinking? :olduhh: ) . Still don't actually remember what a torr is, look up, ah we used to call that 1 mm of mercury. But remember, also from school barometer. 1 arm = 760 mm Hg. So verified neglible for this purpose.

    So 1.7 L is 1.7/22.4 mole, awkward. But it's somewhat less than a tenth. b is quite a lot less, all the others are more, e looks right.

    (Wasn't such a bad school if I can hack that at this distance.).
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