Weight is a force caused by gravity pulling on an object’s mass, so weight changes depending on location. Mass itself is an intrinsic property that stays constant on Earth, the Moon, or in space. In chemistry, mass lets scientists measure exact amounts of reactants and products in a reaction.
The SI base unit for mass is the kilogram (kg), but grams (g) and milligrams (mg) are more practical for everyday lab work. Mass measurements are the foundation of stoichiometry, the branch of chemistry that calculates how much product a reaction will yield. Chemists also use mass to calculate percent composition, which shows what fraction of a compound’s mass comes from each element.
Comparing the actual mass of product to the theoretical mass predicted by calculation gives the percent yield of a reaction. Percent yield is an important measure of efficiency in labs and industry. Chemists distinguish mass from moles as well.
A mole is a counting unit, similar to a dozen, that represents exactly 6.022 × 10^23 particles. Converting between mass and moles requires a substance’s molar mass, found by adding the atomic masses of all atoms in its formula. This conversion connects the mass chemists measure on a balance to the number of particles actually reacting.