Amount of substance concentration, often called simply concentration, is a quantitative measure of the number of atom s per unit volume in a sample of a matter . The matter might be in the solid, liquid, or gaseous state.
The standard unit of concentration is the mole per meter cubed (mol/m 3 or mol · m -3 ). One mole represents approximately 6.022169 x 10 23 particles, usually atoms. Alternatively, concentration can be expressed in moles per centimeter cubed (mol/cm 3 or mol · cm -3 ). To convert from mol/m 3 to mol/cm 3 , multiply by 10 -6 (0.000001). Conversely, multiply by 10 6 (1,000,000).
For a sample of a specific pure elemental substance, the concentration is directly proportional to the density . However, concentration is not proportional to density in general. Consider a cubic meter of space containing one mole of hydrogen atoms. This is about 1/16 the density of a cubic meter of space containing one mole of oxygen atoms, because the mass of an oxygen atom is approximately 16 times the mass of a hydrogen atom. This is true even though the amount-of-substance concentrations of the two parcels are identical.