A chemical equation is a visual representation of a chemical reaction. A typical chemical equation follows the form , where an arrow separates the reactants on the left and the products on the right. The coefficients next to the reactants and products are the stoichiometric values.
On some occasions, it may be necessary to calculate the number of moles of a reagent or product under certain reaction conditions. To do this correctly, the reaction needs to be balanced. The law of conservation states that the quantity of each element does not change in a chemical reaction. Therefore, a chemical equation is balanced when the number of each entry in the equation is the same on both the left and right sides of the equation. Next, we inspect the coefficients of each entry of the equation; the coefficients can be thought of as the amount of moles used in the reaction. What is important here is the reaction stoichiometry, which describes the quantitative relationship among the substances as they participate in the chemical reactions. The relationship between two of the reaction's participants (reactant or product) can be viewed as conversion factors and can be used to facilitate mole-to-mole conversions within the reaction.
For example, we can determine the number of moles of water produced from 2 mol O2. First, the balanced chemical reaction should be written out:
We can see the relationship between O2 and H2O: for every one mole of O2, two moles of H2O are produced. Therefore our ratio is one mole of O2 to two moles of H2O. Now, if we assume abundant hydrogen, and we have two moles of O2, then we can calculate:
The stoichiometric conversion factors are reaction-specific and require that the reaction be balanced. Therefore, the above steps must be followed for every single reaction you encounter.