# 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 and Calculating Mass

• 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.

## An Example Calculation

• 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.

## Important to Note

• 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.