Mass
Definition and meaning of Mass in chemistry.
Mass is the amount of matter in an object or substance. It is usually measured in grams or kilograms. Unlike weight, mass does not depend on gravity, so it stays the same no matter where an object is measured.
In more detail
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.
Key facts
| Field | General Chemistry |
|---|---|
| SI unit | kilogram (kg) |
| Common laboratory unit | gram (g) |
| Key distinction from weight | Mass is constant everywhere; weight varies with gravity |
| Used for | Stoichiometry, percent composition, percent yield |
If you dissolve 5.0 grams of sodium chloride (NaCl) in water, the salt's mass stays exactly 5.0 grams whether the experiment happens on Earth or the Moon. The weight of that same salt would differ between the two locations, since the Moon's gravity is about one-sixth of Earth's.
Frequently asked questions
Is mass the same as weight?
No. Mass measures the quantity of matter and remains the same everywhere, while weight is the gravitational force acting on that mass and changes with location.
Why is mass important in chemistry?
Chemists use mass to measure amounts of reactants and products, calculate stoichiometric ratios, and determine reaction yields and percent composition.
Does mass change during a chemical reaction?
No. The law of conservation of mass says the total mass of reactants equals the total mass of products in a closed system, since atoms are only rearranged, not created or destroyed.