Key concepts in chemistry Study Guide

The empirical formula of a compound represents the simplest whole-number ratio of atoms in that compound. To determine this experimentally, for a substance like magnesium oxide, a known mass of magnesium is reacted with oxygen. The product, magnesium oxide, is then weighed. By comparing the mass of magnesium used and the mass of oxygen that reacted (calculated by subtracting the initial magnesium mass from the final magnesium oxide mass), the moles of each element can be found using their respective atomic masses. The ratio of these moles is then simplified to the smallest whole numbers to yield the empirical formula. This experiment requires careful heating and ensuring complete reaction to obtain accurate results.

The law of conservation of mass states that in any closed system, during a chemical reaction, the total mass of the reactants is equal to the total mass of the products. This means that mass is neither created nor destroyed. In a closed system, such as a sealed flask, all particles involved in the reaction are contained, so any change in mass can be directly attributed to the reaction itself. For example, in a precipitation reaction occurring in a sealed flask, the precipitate forms, but the overall mass of the flask and its contents remains constant because no matter can enter or leave. However, in a non-enclosed or open system, such as a flask open to the atmosphere, the apparent mass can change if gases are involved. If a gaseous product escapes into the surroundings, the measured mass of the flask and its remaining contents will decrease. Conversely, if a gaseous reactant from the surroundings is taken up by the reaction, the measured mass of the flask and its contents will increase. Despite these apparent changes, the law of conservation of mass still holds true because if you accounted for the mass of the escaping or entering gas, the total mass before and after the reaction would remain the same.