Hofmann voltameter

Pure water is a very poor conductor of electricity because there are so few ions in it. However, it can be made to decompose if an electric current is passed through it in a Hofmann voltameter, as in Figure 5.11. 

Figure 5.11 A Hofmann voltameter used to electrolyse water.

Account for the following observations which were made when concentrated sodium chloride solution, to which a little universal indicator had been added, was electrolysed in the laboratory in a Hofmann voltameter. 

When dilute sulphuric acid is electrolysed using platinum electrodes, hydrogen and oxygen are obtained at the electrodes. The apparatus is usually carried out in the Hofmann voltameter, shown on the right. It allows the two gases to be collected. Sulphuric acid has the formula H2SO4. In water it forms ions ... 

The Hofmann voltameter. Note that the volume of hydrogen collected is tv/ice the volume of oxygen. 

Figure 2.4. Diagram of A. W. von Hofmann s voltameter (drawing taken from [W1K5])...

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