A sample of mercury with a mass of 114.0 g was combined with 12.8 g of oxygen gas, and the resulting reaction gave 123.1 g of mercury(II) oxide. How much oxygen was left over after the reaction was complete?

The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. This principle implies that the total mass of the reactants must equal the total mass of the products. In this question, understanding this law is crucial to determine how much oxygen remains after the reaction, as it allows us to account for all the mass involved.

Stoichiometry is the calculation of reactants and products in chemical reactions based on balanced chemical equations. It involves using mole ratios derived from the coefficients of a balanced equation to determine the amounts of substances consumed and produced. In this scenario, stoichiometry helps in understanding how much oxygen reacts with mercury to form mercury(II) oxide, which is essential for calculating any leftover oxygen.

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is used to convert between the mass of a substance and the number of moles, which is necessary for stoichiometric calculations. In this question, knowing the molar masses of mercury and oxygen allows for the determination of how much oxygen was consumed in the reaction, facilitating the calculation of any excess oxygen.