Base units can be used to check the homogeneity of physical equations. In any correct physical equation, each term much have the same unit.
For example, consider the well know relation E = mc2 i.e. energy = mass × (velocity of light)2
Unit of energy = joule = kg m2 s–2
Unit of mass × (velocity of light)2 = kg × (m s–1)2 = kg m2 s–2
The base units for each term are the same; the equation E = mc2 is therefore homogeneous.
Homogeneity is an important condition for a physical equation to be correct. However, it is not the sole condition. The homogeneity test cannot tell us whether there are missing terms (or superfluous terms) in an equation.
The principle of homogeneity of physical equations can be used to derive the units of unknown constants. e.g. for where is a force, is a length and is a speed, the constant has units of N m-2 s = kg m-1 s-1.

One mole (mol) of any substance is defined as that amount of the substance that contains the same number of "elementary entities" as there are atoms in 0.012 kg of carbon-12 (12C).
When the mole is used, the type of "elementary entities" must be specified; these entities may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
The Avogadro constant, usually denoted by L or NA, is equal to the number of atoms in 0.012 kg of carbon-12, i.e. the number of atoms in 1 mol of 12C. The Avogadro Number is given by NA = 6.022 141 79(30) × 1023 mol–1 i.e. mol–1 The atomic mass of a chemical element (or the molecular mass of a compound) is the mass of one mole (in grams). Thus, one mole of 12C has a mass of 12.0 g = 0.0120 kg and contains 6.022 141 79(30) × 1023 atoms of 12C. One mole of O2 has a mass of 32.0 g = 0.0320 kg and contains 6.022 141 79(30) × 1023 molecules of O2. One mole of H2O has a mass of 18.0 g = 0.0180 kg and contains 6.022 141 79(30) × 1023 molecules of H2O.
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Consider the following two measurements of lengths:
L1 = (25 ± 2) mm = (23, 27) mm = {23