Determination of the Empirical Formula of Copper Sulfide
Introduction
The aim of this experiment is to determine the empirical formula of copper sulfide, which is formed when copper and sulfur are heated together. The independent variables are the amount of copper and sulfur, but they will not be varied. The dependent variable is the amount of copper sulfide formed. The controlled variables were among other the size of the beaker and the temperature that the experiment was conducted in.
Materials and Methods
Materials used:
- Copper (Cu)
- Sulfur (S)
- Bunsen burner
- Clay triangle
- Crucible
- Crucible tong
- Scale
- Spoon
First, the copper was weighed on the scale. The copper was then heated together with the sulfur over the Bunsen burner, after which the newly formed copper sulfide was weighed.
Results
First, the Cu was weighed on the scale. Its weight can be found in Table 1: Measured Values.
| Table 1: Measured Values | |
|---|---|
| Substance | Weight [g] |
| Copper (Cu) | 0.49 |
| Copper sulfide | 0.62 |
| Sulfur (S) | 0.13 |
The weight of the substance was measured after the reaction had occurred. The substance, which probably was copper sulfide, weighed 62 g. Because there was initially 49 g of Cu, there must be 13 g of S.
We will use the following formula to calculate the amounts of the mentioned substances in moles:
n = m / M
Where n is the amount of substance in moles, m is the mass of the substance, and M is the molar mass of it. The values of molar mass are according the elements’ respective Wikipedia articles.
| Table 2: Calculations | |||
|---|---|---|---|
| Substance | m (mass) [g] | M (molar mass) [gmol-1] | n (mass) [mol] |
| Copper (Cu) | 0.49 | 63.546 | 0.0077 |
| Sulfur (S) | 0.13 | 32.065 | 0.0041 |
Conclusion
The amount of copper, 0.0077 mol, is approximately double the amount of sulfur, 0.0041 mol. From this, we can conclude that the amount of copper in copper sulfide is twice as large as the amount of sulfur. This means that the empirical formula of copper sulfide is
Cu2S.
Since the amount of sulfur was not exactly half the amount of copper, we had an error somewhere. Probably, not all sulfur reacted with the copper and left some pure sulfur.
Evaluation
The method for making the copper and sulfur react is flawed, because it is not given that all sulfur will react. This is difficult to improve in a standard lab, though.
Another flaw is that the experiment was only conducted once, which means that errors are difficult to spot. We should have performed it at least thrice.
Använder FireStats
For .0077 mole of copper .o41 mole of sulfur is not a 2:1 ratio.
Your site needs to be corrected.
thank you
Jim Prochaska
Comment by jim prochaska — November 5, 2006 @ 11:21 pm
Jim: Since I just started studying I’m not entirely sure, but I believe that you are wrong. 0.0077 mol copper and 0.0041 mol sulfur does not mean that we have half the amount of sulfur and therefore need twice as much — it means that we have half the amount of sulfur, and can’t do anything about it. This means that the formula is written as Cu2S, which symbolizes that there is twice as much copper as there is sulphur.
If you are referring to the error margin, please note that these values are approximate: the experiments were certainly not carried out under accurate circumstances. However, 0.0077 is closer to twice 0.0041 than it is to only 0.004, half it or three times it.
Comment by Tim — November 6, 2006 @ 12:29 am