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Homework Help: Chemistry: Mass to Mass Problem

  1. Nov 6, 2011 #1
    1. The problem statement, all variables and given/known data

    CH4(g) + 3cl2(g) ----> CHCl3(g) + 3HCL(g)
    How many grams of CH4 is needed to produce 50.0 g CHCL3 ?

    For this problem, I'll assume Carbon = 12 amu, Hydrogen = 1 amu, and CL = 35.5 amu.

    3. The attempt at a solution
    See the attachment (A PDF file), for my attempt @ the solution. :cry:
    I think I may have the correct answer; however, I am not positive.

    Attached Files:

    Last edited: Nov 6, 2011
  2. jcsd
  3. Nov 6, 2011 #2
    ok first off, convert your grams of CHCl3 to moles of CHCl3... you should be able to do that. Then from your chemical formula, you are able to find the ratio needed to form a certain reactant. For example, 1 mole of CH4 will form 1 mole of CHCl3. The coefficient of each molecule will tell you how much you need to form the other molecule, if you find a certain amount in moles of CHCl3 then you will need the same amount of CH4 because of the 1:1 ratio. Think of it as the ratio between jam, cheese and bread when making a simple sandwich, you need 1 slice of cheese with 1 slice of jam to be able to make a sandwich (of course you will need 2 slices of bread), now if you have 9 sandwiches at the end, that means you used 9 slices of cheese, from the 1:1 ratio you assume that you must have used 9 slices of jam and from the 1:2 ratio between the cheese and the bread, you assume that you used 18 slices of bread to make 9 sandwiches.. so going back to the problem, once you find moles of CHCl3, you go and use the ratio from the coefficients, then from that you find moles of CH4, finally you can easily go from moles of CH4 to grams of CH4... good luck
  4. Nov 6, 2011 #3
    So was my attempt incorrect? The PDF I attached shows my work.
  5. Nov 6, 2011 #4
    it seems correct to me (the procedure) i dont understand your numbers tho, but it seems that you are following what i said so that must be right..
  6. Nov 6, 2011 #5
    Which numbers are unclear?
    I think I have the procedure down, I want someone to verify the solution if possible though. :shy:
  7. Nov 6, 2011 #6
    yeah that seems correct, i am getting the same thing (6.67 g of CH4)
  8. Nov 6, 2011 #7
    Excellent, thank you! :approve:
  9. Nov 6, 2011 #8
    no problem bro, good luck
  10. Nov 6, 2011 #9


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    Gold Member

    I almost agree with your result but I think you have illegitimately lopped the last significant figure off the calculation, you are given 35.5 for amu of Cl there are 3 Cl atoms so you cannot finish with a whole number denominator.

    Much more important than that is not to make heavy weather as many students do of these calculations. You can do that by imagining the atoms, molecules and their reactions in a very concrete way - draw little diagrams with colours for types of atom which have different weights. Then you see (12 + 4 X 1) g of CH4 goes to make (12 + 1 + 3 X 35.5) g of of CHCl3. After that is is only a question of proportions for what you need to get 50g of the later. Admittedly simple proportions have not been grasped confidently by all students.

    In this case reaction is in a sense is simple, the C atoms in reactant go only to C atoms in product, in other cases that might not be so, so you have to watch it.
  11. Nov 6, 2011 #10
    That's true, I was less worried about the sig. figs here, I just wanted to make sure I grasped the concept. It's actually 119.5, I just rounded to 120 to make it easier.

    I never would have thought of the problem in that way, I'll be sure to try to spot such things in the future. :smile:
  12. Nov 6, 2011 #11


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