Composition of Water
Water can be analyzed, that is, broken into its components, by electrolysis, which
is discussed in Chapter 5. This process shows that the composition of water by
volume is 2 parts of hydrogen to 1 part of oxygen. Water composition can also be
arrived at by synthesis. Synthesis is the formation of a compound by uniting its
components. Water can be made by mixing hydrogen and oxygen in a eudiometer
over mercury and passing an igniting spark through the mixture. Again the ratio of
combination is found to be 2 parts hydrogen: 1 part oxygen. In a steam-jacketed
eudiometer, which keeps the water formed in the gas phase, 2 volumes of
hydrogen combine with 1 volume of oxygen to form 2 volumes of steam.
Another interesting method is the Dumas experiment pictured in Figure 27.
Data obtained show that hydrogen and oxygen combine to form water in a ratio of
1 : 8 by mass. This means that 1 gram of hydrogen combines with 8 grams of
oxygen to form 9 grams of water.
Figure 27. Synthesis of WaterSome sample problems involving the composition of water are shown
below.
Typical Problem (by Mass)
An electric spark is passed through a mixture of 12 grams of hydrogen and 24
grams of oxygen in the eudiometer setup shown. Find the number of grams of
water formed and the number of grams of gas left uncombined.
Since water forms in a ratio of 1 : 8 by mass, to use up the oxygen (which by
inspection will be the limiting factor since it has enough hydrogen present to react
completely) we need only 3.0 grams of hydrogen.