Final answer:
The small whole number that appears in front of a formula in a chemical equation is called a coefficient, which represents the number of molecules, atoms, or formula units involved in the reaction and establishes the mole ratio of reactants and products.
Step-by-step explanation:
The small whole number that appears in front of a formula in a chemical equation is called a coefficient. Coefficients are used to indicate how many molecules, atoms, or formula units of a substance are involved in a reaction. For example, in the balanced equation 2H2O, the coefficient is '2', which tells us that two molecules of water are being considered. It's important to note that a coefficient of 1 is typically omitted in chemical equations, and you should never change the subscripts in a chemical formula; you can only balance equations by changing the coefficients. These coefficients are crucial as they express the mole ratio of reactants and products in a balanced equation, ensuring that the law of conservation of mass is satisfied.