Lesson 2

Mass

Grams measure the property of matter which we call mass. In chemistry you can also call it weight. There is a distinction between mass and weight. Let me dwell on that for a moment, even though it is something that chemists tend to ignore.

Mass and Weight

Mass measures the amount of matter in an object. It is related to inertia and to weight. Weight is the gravitational attraction between the earth and the mass of the object. The object's mass is attracted to the earth and the strength of that attraction is called the weight. Something can have mass and still be weightless by being so far away from the earth that the force of gravitational attraction is negligible.

Since most of our chemical reactions are done on or near the earth's surface, we don't have to worry about changes in weight due to varying distance from earth. Here on earth, if two things have the same mass, they will also have the same weight. If you took a billiard ball or a chemical sample to the moon or into outer space, its weight would be less but its mass would remain the same. Chemists usually refer to the weight rather than the mass. We treat mass and weight essentially the same although technically we should call it mass. If you would like to know more about the distinction between mass and weight, check your textbook or ask the instructor.

Metric Units of Mass

For weight or mass, our standard unit is the gram. This is the unit we will be using most. If we wanted to measure very small amounts of something, we would measure the mass in milligrams, in thousandths of grams. If we wanted to measure heavier objects, we would use the kilogram which is 1,000 times a gram. The SI of International System uses the kilogram as its standard rather than the gram.