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Homework Help: Ideal gas law calculation

  1. Oct 28, 2014 #1
    1. The problem statement, all variables and given/known data

    Determine how many grams of Mg must react with HCl in order to produce 40 mL of hydrogen gas at 22 degrees Celsius and 1.02 bar. Also, If the Mg ribbon weighs 0.836 mg/mm, how many mm of the ribbon will you require?

    2. Relevant equations


    Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)

    3. The attempt at a solution

    n= (1.00 atm)(0.04 L) / (0.0821 L*atm/mol*K)(295.15 K)
    n= 0.04/24.232 = 0.0016507098 mol * 24.3050 g/mol = 0.040 g Mg

    40 mg x 1mm / 0.836mg = 47.85mm

    Correct? Thanks for the help!
  2. jcsd
  3. Oct 28, 2014 #2


    User Avatar

    Staff: Mentor

    Result looks OK to me, calculations are rather confusing.

    Let's say it is OK, although 1 atm is not exactly equal to 1.02 bar.

    And that's where you have lost me. What is 0.04 and where did you get it from? What is 24.232 and where did you get it from? They have no units, so I would have to guess what they are, plus, they don't follow from the previous line.
  4. Oct 28, 2014 #3

    Yes, I see what you are saying. I will add the units and be more specific with my pressure conversion.

    the 0.04 comes from 0.04L x 1.00 atm and the 24.232 comes from (0.0821 L*atm/mol*K)*(295.15 K). Thanks for your help! :)
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