The Distinction Between Mass and Weight

It is important to understand the difference between mass and weight. Mass is an invariant measure of the quantity of matter in an object. Weight is the force of attraction between an object and its surroundings, principally the earth. Because gravitational attraction varies with geographical location, the weight of an object depends on where you weigh it. For example, a crucible weighs less in Denver than in Atlantic City (both cities are at approximately the same latitude) because the attractive force between the crucible and the earth is smaller at the higher altitude of Denver. Similarly, the crucible weighs more in Seattle than in Panama (both cities are at sea level) because the Earth is somewhat flattened at the poles, and the force of attraction increases measurably with latitude. The mass of the crucible, however, remains constant regardless of where you measure it. Weight and mass are related by the familiar expression:

w = mg

where w is the weight of an object, m is its mass, and g is the acceleration due to gravity. A chemical analysis is always based on mass so that the results will not depend on locality. A balance is used to compare the mass of an object with the mass of one or more standard masses. Because g affects both unknown and known equally, the mass of the object is identical to the standard masses with which it is compared.

The distinction between mass and weight is often lost in common usage, and the process of comparing masses is usually called weighing. In addition, the objects of known mass as well as the results of weighing are frequently called weights. Always bear in mind, however, that analytical data are based on mass rather than weight. Therefore, throughout this text, we will use mass rather than weight to describe the quantities of substances or objects. On the other hand, for lack of a better word, we will use “weigh” for the act of determining the mass of an object. Also, we will often say “weights” to mean the standard masses used in weighing.