Calculating Masses of Reactants or Products from a Balanced Chemical Equation

Understanding Chemical Equations

A chemical equation represents a chemical reaction. Reactants are listed on the left and products on the right, separated by an arrow.
Balancing chemical equations is essential; it ensures the law of conservation of mass is obeyed.
Moles represent the amount of a substance. One mole of any substance contains Avogadro’s number (6.02 x 10^23) of entities (atoms, molecules, ions etc.).

Stoichiometry is the calculation of quantities of reactants and products in a chemical reaction.
Molar mass (M) is the mass of one mole of a substance. It is equal to the relative formula mass of that substance in grams.
Use molar ratios from the balanced chemical equation to calculate the mass of reactants or products.

For instance, in the reaction 2H2 + O2 -> 2H2O, twice as many moles of hydrogen react with one mole of oxygen to form two moles of water.
If we know the mass of hydrogen and want to find the mass of water produced, we would calculate the moles of hydrogen first using n = m/M, then use the molar ratio to find the moles of water, and finally convert back to mass.

All calculations assume that the reaction goes to completion.
If a reactant is not completely converted into product or if there are side reactions, the actual amount of product will be less.
If the reaction does not go to completion or if the balanced equation is incorrect, calculations may not be accurate.
Always remember the law of conservation of mass: “matter cannot be created or destroyed”, the total mass of the reactants always equals the total mass of the products.