[evankd2]

I am a freshman college student and I just did this experiment today and I am not sure what the sources of error could be.

Elemental Analysis: Determination of the percent copper in an unknown.

In the experiment the mass of the sample was determined, and it was mixed into a water and HCL solution until it was completely dissolved. Magnesium turnings were added to the solution and mixed until the solution was clear.  The solid was seperated by vacuum filtration, and the sample was dried.  The mass of the sample was obtained and the data was used to get the % copper, the mols copper, the MM of the unknown and then to identify the unknown.  Finally % error was calculated.

I am still not very sure of what sources of error are in experiments such as this, although I know that they cannot be anything that you could have done wrong.  Any help would be apprecitated. Thanks.

[Borek]

Strange combination - copper and HCl.

What are usual sources of error in gravimetric analysis?

[Arkcon]

Try and look at each measurement you made, is there an error figure printed on the instrument?  If not, is there a similar type of measuring device that has +/- values printed on it?  Even if you don't use that particular device or instrument, you might be able to safely conclude that your own measurement has an error associated with it.  Not the same one, but at least you'll know where to look.  Back in the old days, we started our lab career making repeated measurements, to see what are error was, did you do that?

[evankd2]

Our lab instructor never went over sources of error for these kinds of experiments, and honestly I am a little confused over what sources of error in experiments are.

[Arkcon]

Yeah, I remember college classes where there's a question on the practical that isn't mentioned before.  They may be trying to subtlety coax you to ask the T.A., seek out the prof's office hours, look ahead in your textbook, or intuit the answer.  Try to guess, we'll try to take it from there.

[evankd2]

Sources of error could be:
-loss of some of the solid on the sides of the filter/filter paper when filtering
-measuring the mass when the temp. had not returned exactly to normal after drying