Multiply the coefficient of a molecule times the subscript of each of its elements to find the number of atoms that the molecule contributes to the reaction to balance the chemical equation. If there is no coefficient or subscript, use a value of 1. For example, the reaction Zn + HCl -> ZnCl2 + H2 is unbalanced, as there are two hydrogen and two chlorine atoms on the right side, and the left side has only one of each. Add a coefficient of 2 in front of HCl to balance it -- Zn + 2HCl -> ZnCl2 + H2.
Add up all of their atomic weights to find the molecular weight of the entire molecule of each reactant. You can find the atomic weights of the elements in the molecule by looking on the periodic table of elements. If any of the elements has a subscript, multiply the atomic weight of that element by the subscript before adding.
Divide the number of grams of the molecule by the molecular weight to find the number of moles. For example, the atomic weight of zinc is 65.4 grams/mole, so if you have 2.5 grams of zinc, the number of moles is 2.5/65.4 = 0.038.
Divide the number of moles of each reactant by its coefficient and take the smallest of those values to determine the limiting number of moles. For example, the coefficient of Zn is 1, so 0.038/1 = 0.038. For HCl, 0.5/2 = 0.25. Therefore, the limiting number of moles is 0.038 because it is smaller. That means that there is more hydrochloric acid than is necessary, so the quantity of zinc determines the quantity of the products.
Multiply the limiting number of moles times each of the product's coefficients to calculate the number of moles of each of the products. For example, the coefficients for both ZnCl2 and H2 are 1, so the number of moles of each is equal to the number of moles of zinc, which is 0.038.