Final answer:
In a chemical equation, an absent coefficient in front of a chemical formula indicates that the understood number is '1', signifying one mole of the substance in the reaction.
Step-by-step explanation:
When there is no number in front of a chemical formula in a chemical equation, it is understood that the number is '1'. In a balanced chemical equation, coefficients are used to indicate the mole ratio in which the reactants combine to form products. If a chemical formula has no coefficient written before it, we assume the coefficient is 1. This means one mole of the substance is involved in the reaction. Coefficients are particularly important as they reflect the relative numbers of molecules or formula units in the reactants and products.