Mass is not conserved in chemical reactions. The fundamental conservation law of the universe is the conservation of mass-energy. This means that the total mass and energy before a reaction in a closed system equals the total mass and energy after the reaction. According to Einstein's famous equation, E = mc2, mass can be transformed into energy and energy can be transformed into mass. This is not some exotic process, but in fact happens every time there is a reaction. Mass is therefore never conserved because a little of it turns into energy (or a little energy turns into mass) in every reaction. But mass+energy is always conserved. Energy cannot be created out of nothing. It can only be created by destroying the appropriate amount of mass according to E = mc2. Between mass and energy, energy is the more fundamental property. In fact, modern physicists just consider mass an alternate form of energy. For this reason, they don't usually call it the "Law of Conservation of Mass/Energy" but rather call it the "Law of Conservation of Energy" with the implication that this statement includes mass.