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Chemical reactions and amounts of substances

It is customary to consider substances as composed of the “building blocks” of matter, the elements. Consider some reactions involving any two of the three elements hydrogen, oxygen, and chlorine, each time involving two of them. If we start with a unit amount of substance of hydrogen gas having a mass of 2 grams, we find that 16 g of oxygen gas will completely react with the hydrogen to form water. However, exactly twice that amount of oxygen gas, i.e., oxygen with a mass of 32 g, will react completely with 35.5 g of chlorine. Finally, twice as much chlorine, i.e., 71 g, will use up 2 g of hydrogen gas. Fig. 1 shows these examples written in the language of chemical reaction equations.

Interpretation

We see that the mass of a certain quantity of an element cannot serve as a natural measure of amount of substance. It appears that 71 g of chlorine are chemically equivalent to 2 g of hydrogen. Therefore, it is necessary to introduce a new measure of amount of substance. If we count 2 g of hydrogen gas as one unit of amount of substance, called 1 mole, the reactions tell us that 1 mole of hydrogen gas (H2) has a mass of 2 g, 1 mole of oxygen gas (O2) has a mass of 32 g, while the mass of 1 mole of chlorine (Cl2) is 71 g. The mass of one mole of amount of substance is called the molar mass of the substance.

According to the particle model of the structure of substances, equal amounts of substance correspond to equal numbers or particles.