The weight of an object is the force of gravity on the object and may be defined as the mass times the acceleration of gravity, w = mg. Since the weight is a force, its SI unit is the newton (1N = 1kg x 1s²).

If an object has a mass of 1 kg on th Earth, it would have a mass of 1 kg on the Moon, even though it would weigh only 1/6 as much.

The standart for mass is a piece of metal kept at the International Bureau of Weights and Measures in Sevres, France. This object is called the International Prototype Kilogram. Its mass is defined as one kilogram. The SI standart of mass is the kilogram (kg). However, the kilogram is too large a unit for convenient use in the chemical laboratory. For this reason, the gram (g), 1/1000 of a kg, is commonly used.

An instrument used to determine the mass of an object by comparing the object's mass to known mass is called a balance. To compare this masses, first place the object with unknown mass on the balance pan. Then add known masses to the beams until the masses are equal.

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