# Homework Help: Law of Constant Composition

1. Sep 13, 2010

### priscilla98

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. Sep 13, 2010

### Bohrok

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.

3. Sep 13, 2010

### priscilla98

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

4. Sep 13, 2010

### Bohrok

Do you know what compound is made with zinc and sulfur?

5. Sep 13, 2010

### priscilla98

Wouldn't it make zinc sulfide?

6. Sep 13, 2010

### Bohrok

And what's the formula for zinc sulfide? Then write the complete equation for the reaction.

7. Sep 13, 2010

### priscilla98

Is the formula ZnS2? I'm thinking since its sulfide then it would be bottom squared for S.

8. Sep 13, 2010

### Bohrok

Not quite, think of what the charges are for Zn and S as ions.

9. Sep 13, 2010

### priscilla98

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?

10. Sep 13, 2010

### Bohrok

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?

11. Sep 13, 2010

### priscilla98

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?

12. Sep 13, 2010

### Bohrok

You got the answer, good job.

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.