# Mass, Density, and Volume - Key terms

### ATOMIC MASS UNIT:

An SI unit (abbreviated amu), equal to 1.66 · 10 −24 g, for measuring the mass of atoms.

### AVERAGE ATOMIC MASS:

A figure used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample. The average atomic mass of carbon, for instance, is 12.01 amu. If a substance is a compound, the average atomic mass of all atoms in a molecule of that substance must be added together to yield the average molecular mass of that substance.

A figure, named after Italian physicist Amedeo Avogadro (1776-1856), equal to 6.022137 × +023 . Avogadro's number indicates the number of atoms, molecules, or other elementary particles in a mole.

### DENSITY:

The ratio of mass to volume—in other words, the amount of matter within a given area. In the SI system, density is typically expressed as grams per cubic centimeter (g/cm 3 ), equal to 62.42197 pounds per cubic foot in the English system.

### MASS:

The amount of matter an object contains.

### MATTER:

Physical substance that occupies space, has mass, is composed of atoms (or in the case of subatomic particles, is part of an atom), and is convertible to energy.

### MILLILITER:

One of the most commonly used units of volume in the SI system of measures. The milliliter (abbreviated mL), also known as a cubic centimeter (cc), is equal to 6.10237 · 10 −2 cubic inches in the English system. As the name implies, there are 1,000 milliliters in a liter.

### MOLAR MASS:

The mass, in grams, of1 mole of a given substance. The value in grams of molar mass is always equal to the value, in atomic mass units, of the average atomic mass of that substance: thus, carbon has a molar mass of 12.01 g, and anaverage atomic mass of 12.01 amu.

### MOLE:

The SI fundamental unit for "amount of substance." A mole is, generally speaking, Avogadro's number of atoms, molecules, or other elementary particles; however, in the more precise SI definition, a mole is equal to the number of carbon atoms in 12.01 g of carbon.

### SPECIFIC GRAVITY:

The density of an object or substance relative to the density of water; or more generally, the ratio between the densities of two objects or substances. Since the specific gravity of water is 1.00—also the density of water ing/cm 3 —the specific gravity of any substance is the same as the value of its own density in g/cm 3 . Specific gravity is simply a number, without any unit of measure.

### VOLUME:

The amount of three-dimensional space an object occupies. Volume is usually expressed in cubic units of length—for instance, the milliliter.

### WEIGHT:

The product of mass multiplied by the acceleration due to gravity (32 ft or 9.8 m/sec 2 ). A pound is a unit of weight, whereas a kilogram is a unit ofmass.