Learn the basics about the law of constant composition and how to apply it.
The law of constant composition states that in a given chemical compound, all samples are made up of the same elements combined in the same proportions.
So what determines the ways in which elements combine? Well, it depends upon their positions in the periodic table and on how many bonds they can form.
The law of constant composition is a useful tool for defining compounds, but it has its limits.
Some compounds are non-stoichiometric and their elemental composition varies between samples. These compounds have similar ratios of the same kinds of elements, but not exactly the same ratios. One such compound is iron selenide, FeSe. It forms close to Fe0.9Se. This non-stoichiometry gives rise to magnetic properties: iron selenide is a low temperature superconductor!
So in general, the law of constant composition helps us to define compounds and differentiate between them: all samples of one compound contain the same ratios of the same kinds of elements. Although it has limited applications in large compounds, it holds true for small, molecular compounds like H2O. We have also learnt how to convert formulae into elemental content (mass percentages) - a useful tool of analytical chemistry.
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