The text has been rewritten to be less than 500 words while maintaining the main ideas from the original. Here is the rewritten text:

The mole is a convenient unit of measurement for expressing large numbers of atoms or molecules. The identity of a substance is defined by the types of atoms or ions it contains, as well as the quantity of each type. For example, water and hydrogen peroxide are alike in that they consist of hydrogen and oxygen atoms, but hydrogen peroxide has twice as many oxygen atoms. The mole provides a link between easily measured macroscopic properties and fundamental microscopic traits.

The formula mass of a substance is the sum of the average atomic masses of all the atoms represented in its chemical formula. For covalent compounds, the formula mass represents the molecular mass. The average mass of a chloroform molecule is 119.37 amu, and that of aspirin is 180.15 amu.

For ionic compounds, the formula mass is calculated in the same way as for covalent compounds. The average masses of sodium and chloride atoms were used to compute the formula mass of sodium chloride, which is 58.44 amu.

In calculating the formula mass of an ionic compound, it is acceptable to use the average masses of neutral atoms rather than the masses of charged atoms or ions. The missing or additional electrons can generally be ignored when computing the mass of an isolated ion, as their contribution to the overall mass is negligible and reflected only in the nonsignificant digits that will be lost when the computed mass is properly rounded.

In summary, the mole is a convenient unit of measurement for expressing large numbers of atoms or molecules, and the formula mass of a substance is the sum of the average atomic masses of all the atoms represented in its chemical formula.