Law of Definite Proportions 2


Lead iodide sediment formation reaction

If you continue to add the lead nitrate solution to a certain amount of potassium iodide solution, the amount of sediment produced will continue to increase at the beginning and will no longer increase at some point.
This is because all potassium iodide solutions react, and even if the lead nitrate solution is continuously added, no further reaction occurs. So, we can draw the following conclusions:

  1. There is a certain ratio between the reacted potassium iodide and lead nitrate and the formed lead iodide and potassium nitrate.
  2. There is a certain mass ratio between the constituent substances that make up lead iodide.

The law of definite proportions

The law of definite proportions is that “the mass ratio of the constituent elements constituting a compound is always constant.” French chemist Proust proposed the law of definite proportions in 1799.

At the time, his friend and chemist Berthollet opposed Proust’s claim and argued that the compound’s composition was not constant. He said that if making the same compound or the amount of sample is different, the composition will vary in many ways, and Proust’s proposal will be made only in special cases. Berthollet analyzed the sulfides of copper or oxides of iron and insisted that the composition ratio was not constant. He did not know that there were more than two types of sulfides of copper or oxides of iron, so Proust’s proposal was finally proved correct.