Determination of the empirical formula for copper sulphide
In this experiment we will try to find out the empirical formula for copper sulphide. To be able to estimate the empirical formula of copper sulphide we need to know the mass in grams of copper separately, and then the mass in grams of copper sulphide after a reaction between the elements. From this data we should be able to calculate the empirical formula of copper sulphide.
The result may vary as some sulphur can get attached on the copper sulphide during the reaction, also, the mass in grams can be slightly incorrect if there is some dirt on the scale which weigh the substance on.
Equipment
For this experiment we will require the following equipment, materials and chemicals; Ring stand with clamp, a gas burner, tongs, a large test tube with iron thread, eye protection, a copper thread and powdered sulphur.
Procedure
Firstly we take about 20 cm of copper thread and measure the mass in grams as accurately as possible on the scale. We then add about 2 cm height of sulphur to a test tube and attach the copper by and iron thread, hanging above the sulphur in the test tube.
Now, we will shut off the light and start to heat the tube gently using a gas burner, until the sulphur gas/liquid mixture completely covers the copper thread. As we have turned off the light we can see the reaction happening inside the test tube.
After the reaction has taken place we will remove the copper thread together with the iron and determine that all the sulphur is burning completely, the thread has to cool down now until we can measure its mass in grams, it is now copper sulphide.
The data that we have collected should now be sufficient to determine the empirical formula for copper sulphide.
Results
The first observation we can make is that during the reaction between copper and sulphur. There was a short but intense glowing red light which determined that the reaction took place.
This is what our collected data looked like after we had weighed the copper and the copper sulphide;
| Element/Compound | Mass in grams |
| Copper | 0.87 grams |
| Copper Sulphide | 1.27 grams |
From this data we can calculate the empirical formula of Copper Sulphide;
Mass of Sulphide combining with copper = 1.27-0.87 = 0.4 grams
Amount of copper = 0.87/63.5 = 1.37*10-2 mol
Amount of sulphur = 0.4/32 = 1.25*10-2 mol
Simplified ratio = 1.37*10-2 mol/1.25*10-2= 1.1
= 1.25*10-2 mol/1.25*10-2 = 1
Ratio of copper sulphide = 1.1: 1
Calculated empirical formula Cu11S10
Conclusion and Evaluation
If this experiment had been done without error the empirical formula for copper sulphide should have been Cu2S. Instead we got Cu11S10. Obviously we have done an error during our experiment. One of the most obvious mistakes which we could observe was that sulphur which had not reacted with the copper was attached to it. This gave us an incorrect mass of the compound copper sulphide and consequently an incorrect mass of sulphur that had reacted with the copper.
To be able to get a more accurate result, we should not have used as much sulphur as we did in the test tube, the copper hanging to close to the sulphur can be one reason that so much sulphur was attached to the copper sulphide.
Summary
In this experiment we have made an effort to determine the empirical formula for copper sulphide. We accomplished this by weighing firstly the coppers mass in grams and secondly by weighing the mass of the same copper thread after a reaction with sulphur. Sulphur is now added to the thread and after collecting this data we have calculated the empirical formula. Some errors occurred during the experiment and the result was incorrect, therefore I have stated how we could have improved the result by adding less sulphur to the test tube.
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