Although mass is a property of matter we can conveniently measure in the laboratory, the coefficients of a balanced chemical equation are not directly interpreted on the basis of mass. Explain why.

In a certain chemical reaction, the bond between atoms will break and the atoms will rearrange. So the balance equation is related to the number of atoms. For example, 2H2 + O2 = 2H2O, the atom number must be the same at both sides of the equation instead of the mass of reactants and production. And because of the different atom mass, the mass of matter is not convenient to compare the atom number relationship.

The coefficients of a balanced chemical equation means number of particles (molecules or unit formulas) not mass.

The law of definite proportion states that a compound is constituted by the same elements in the same proportion regardless the total amount.

A chemical formula states the ratio of the atoms in a compound as number of atoms, not as mass or weight.

For example, the formula Na₂O means that the compound is formed in a ratio of 2 atoms of Na to 1 atom of O. (Not two grams of Na to one gram of O).

Then, the chemical equation Na + O₂ → Na₂O when balanced results in

4Na + O₂ → 2Na₂O.

The coefficients are used to balance the number of atoms in the compounds of each side: 4 atoms of Na in the reactant side and 4 atoms of Na in the product side, 2 atoms of O in the reactant side and 2 atoms of O in the reactant side.

So, the coefficients mean number of unit formulas, not mass.