Law of Constant Composition

  1. 1. The problem statement, all variables and given/known data
    4.0 g of sulfur and 10.0 g of zinc are heated together, how much zinc will remain unreacted?

    2. Relevant equations

    3. The attempt at a solution

    If these two elements are heated together, wouldn't the total mass (g) be 14.0 by adding the amount of sulfur and zinc together. I'm thinking half of the zinc would remain unreacted
  2. jcsd
  3. This is a limiting reagent problem; are you familiar with those?

    The sulfur and zinc will form one compound. And since the problem already tells you that not all the zinc will react, you'll find out exactly how much will react, based off of the grams of sulfur you have.
  4. Well, i just started ap chem and i have no clue about the law of constant composition. Therefore, as you say, not all of the zinc will react, but how exactly do you find out how much zinc will not react
  5. Do you know what compound is made with zinc and sulfur?
  6. Wouldn't it make zinc sulfide?
  7. And what's the formula for zinc sulfide? Then write the complete equation for the reaction.
  8. Is the formula ZnS2? I'm thinking since its sulfide then it would be bottom squared for S.
  9. Not quite, think of what the charges are for Zn and S as ions.
  10. ok the charge for Zn is + 2 and the charge for S is - 2. So wouldn't the charges cross out and it would be just ZnS?
  11. Right, so the equation is Zn + S → ZnS.

    Since some of the zinc remains unreacted, that means all of the sulfur will react with part of the zinc.
    Now given 4.0 g S, how could we start finding out how much Zn will react with that?
  12. Ok i think im starting to understand it.

    Moles of Zn = 10 g./65.4 g/mole = 0.153 moles

    Moles of S = 4g/32.1 g/mole = 0.125 moles

    Per the reaction Zn and S react in a 1:1 ratio so when the 0.125 mole of S are consumed the reaction will stop and 0.153-0.125 = 0.028 moles of Zn will remain.

    0.028 mole(65.4g/mole) = 1.82 grams of Zn remain

    But would it be the same answer if they ask how much sulfur will remain unchanged?
  13. You got the answer, good job. :smile:

    Asking how much sulfur will remain unchanged wouldn't make sense since all of it (theoretically) would be used up because it is the limiting reagent and there is excess zinc.
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