Standard 4e Preknowledge
4e) Students know how to calculate the masses of reactants and products in a chemical reaction from the mass of one of the reactants or products and the relevant atomic masses.
CALIFORNIA FRAMEWORKS SUMMARY: Atoms are neither created nor destroyed in a chemical reaction. When the chemical reaction is written as a balanced expression, it is possible to calculate the mass of any one of the products or of any one of the reactants if the mass of just one reactant or product is known. Students can be taught how to use balanced chemical equations to predict the mass of any product or reactant. Teachers should emphasize that the coefficients in the balanced chemical equation are mole quantities, not masses. Here is an example:
How many grams of water will be obtained by combining 5.0 grams of hydrogen gas with an excess of oxygen gas, according to the following balanced equation?
2H2 + O2 → 2H2O
This calculation is often set up algebraically, for example, as
and can be easily completed by direct calculation and unit cancellation (dimensional analysis). Students should learn to recognize that the coefficients in the balanced equations refer to moles rather than to mass.
