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Understanding the chemical formula of a substance is fundamental in chemistry. It provides essential information about the types and quantities of atoms that constitute a molecule. This code, such as H2O for water or C6H12O6 for glucose, represents the basic units of chemical compounds. The subscript numbers indicate the number of atoms of each element present.

When calculating the percentage composition of a substance, the chemical formula informs you of the stoichiometry, which is the ratio of elements that come together to form compounds. This ratio is necessary to determine how much of each element contributes to the total mass of the compound. Without this information, it wouldn't be possible to calculate the percentage composition accurately.

Molar mass is a measure of the mass of one mole of a given substance and is expressed in grams per mole (g/mol). It's equivalent to the sum of the atomic masses of each element in a chemical compound, multiplied by the number of atoms of that element in the formula.

Thus, to calculate the molar mass of a compound, as in Step 2 of the example problem, you look up the atomic masses on the periodic table for each element. For example, for water (H2O), you'll find that hydrogen has an atomic mass of approximately 1.008 g/mol and oxygen has an atomic mass of about 16.00 g/mol. Since there are two hydrogens in water, their contribution is twice their individual atomic mass, resulting in a molar mass calculation of approximately 18.02 g/mol for water.

At the heart of each calculation involving a chemical formula is the atomic mass, which is the weight of a single atom of an element. It is often referred to as the 'relative atomic mass' because it is measured relative to the mass of carbon-12, which is set at exactly 12 atomic mass units (u).

The atomic mass of each element is a weighted average that reflects the various isotopes and their abundance in nature, and is given in atomic mass units or Daltons. These values are listed on the periodic table and are used in combination with the chemical formula to calculate the molar mass of compounds.

Stoichiometry is the section of chemistry that involves the calculation of reactants and products in chemical reactions. It's based on the law of conservation of mass and the concept that matter is neither created nor destroyed in chemical reactions.

For the calculation of percentage composition, stoichiometry comes into play by interpreting the chemical formula in terms of moles, which can then be used to determine mass ratios. By dividing the molar mass of each element in the compound by the total molar mass, and then multiplying by 100, as detailed in Step 4, you attain the percentage composition, which reveals the proportional mass contributions of each element to the overall compound.