Mole Concept and Molar Mass

The number of particles that makes up even a small amount of a substance is so large that chemists use a special number to describe them. A mole is the amount of a substance that contains as many particles as 12 grams of pure carbon-12, equal to $6.023\times10^{23}$ (Avogadro's number) particles. For example, one mole of neon (Ne) contains $6.023\times10^{23}$ neon atoms, and one mole of hydrogen gas (H₂) contains $6.023\times10^{23}$ hydrogen molecules. Moles are useful in chemistry for describing the amounts of different substances that take part in a chemical reaction. However, along with the concept of the mole, it is necessary to understand the ratios of different types of atoms.

A molecular formula indicates the number of atoms of each type of element in a molecule. An empirical formula indicates the ratio of elements found in a compound in the lowest whole number possible. For example, the molecular formula of glucose (C₆H₁₂O₆) shows that each glucose molecule has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. The empirical formula of glucose (CH₂O) shows that each molecule has twice the number of hydrogen atoms as carbon atoms and oxygen atoms. Similarly, the molecular formula of ethane (C₂H₆) shows that each molecule has 2 carbon atoms and 6 hydrogen atoms. Its empirical formula, CH₃, shows that each molecule has three times as many hydrogen atoms as carbon atoms.

In a balanced chemical equation, the coefficients of the reactants (on the left side) and the coefficients of the products (on the right side) indicate the ratios of moles of the species participating in the reaction. This ratio of moles is called the mole ratio of reactants to products. Consider the reaction of copper oxide (CuO) and hydrochloric acid (HCl). The coefficients show that one mole of copper oxide reacts with two moles of hydrochloric acid to produce one mole of copper chloride, CuCl₂, and one mole of water, H₂O. Using these coefficients, the number of moles of any reactant and product involved in the reaction can be determined as long as the number of moles of any of the four is known. Describing a reaction using the masses of the reactants and products requires a different unit. Molar mass is the mass of one mole of the atoms, molecules, or formula units that make up a substance. It is measured in grams per mole (g/mol). The molar mass of each element in its atomic state is equal to its atomic mass, which is found in the periodic table. For example, the periodic table shows that the atomic mass of sodium (Na) is 22.99 atomic mass units (amu). So, its molar mass is 22.99 g/mol. The atomic mass of chlorine is 35.45 amu. So, the molar mass of chlorine in its atomic state is 35.45 g/mol. The molar masses of individual elements can be used to calculate the molar mass of a compound.

One of the most important stoichiometric calculations in chemistry is the conversion of moles of a substance to grams and vice versa. This relationship can be written as a conversion formula. This formula can be used to calculate either moles or mass of a substance, depending on the variables present. A diagram of the three factors involved in the formula is useful for remembering the formula.