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Homework Help: Reacting masses Mg and Cl2

  1. Apr 18, 2017 #1


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    1. The problem statement, all variables and given/known data

    Trying to help my son with this chemistry question.

    2. Relevant equations
    Given in the problem statement

    3. The attempt at a solution
    We think we have the first part right:

    24g of Magnesium is one mol.
    One mol of CL is 35.5g but we need one mol of CL2 so 2*35.5=71g
    Then by conservation of mass 24+71=95g of Magnesium Chloride.

    We definitely have a problem is with the last part of the question...

    The multiple choice answers are 396g, 119g or 100g

    Attempt 1:
    100g of CL2 is used instead of 71g so we scaled up the magnesium as follows:
    24 * 100/71 = 33.8g
    So now the amount of magnesium chloride produced should be 100g+33.8g = 133.8g. But that's not an option.

    Attempt 2

    The problem doesn't actually mention changing the magnesium. In that case the amount of Magnesium chloride produced would be unchanged at 95g and some of the Chlorine would remain unreacted. But 95g isn't a valid answer either.


    396g seems way too high to be the answer. 100g is obviously wrong if 100g of Chlorine alone is used. So 119g seems like the most likely answer but how?
  2. jcsd
  3. Apr 18, 2017 #2


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    Your calculations are correct - 134 g is a right answer. There is something wrong with the test.
  4. Apr 18, 2017 #3


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    Thanks. I worked out that if it had said 100g of magnesium instead of chlorine then the output would be 396g so perhaps that's the error in the question.
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