Expert: Leo Carr - 5/10/2006

Question
When a copper/magnesium cell was set up (1M copper(II)sulphate solution and 1M magnesium sulphate solution are used as electrolytes respectively), I found the voltage measured was 1.03 V. However, when I tried to connect two copper/magnesium cells in series, the voltage was only 1.74 V. I have already repeated the experiment several time, but the results are the same. Would you mind to tell me why the voltage is not 2.06 V?
We are looking forward to your kind reply! Thanks!

Answer
1.  Are sure the concentrations are close to 1M?
2. Ensure your salt bridge is wet if using filter paper or ensure it is still there?
3.  Is your electrodes clean; Cu and/or Mg.  

Looking at the data it is not all that off.  You report a percent error of only 18%.  What is your calculation for the cell EMF based off the potentials of your half reactions?

I have done labs such as this and my results have been very good.  I am not sure how much depends upon your volt meter quality.

Hope this helps.