Avogadro's number, 6.022×1023
, is the number of particles of a substance per mole of that substance. A mole is the amount of a substance that contains as many particles as 12 grams of pure carbon-12. Molar mass, then, is the mass of one mole of a substance. It can be used to calculate the number of moles or molecules of a given substance when the mass of the substance is known. A compound's molecular formula shows the number of atoms of each element present in the compound, whereas its empirical formula shows only the ratio of elements in the compound. Percent composition is the percentage by mass of each element in a compound. Percent composition and molar mass can be used to determine a compound's molecular and empirical formulas. Molarity describes the concentration of an aqueous solution in which materials are dissolved in water. Other measurements, such as mass percent or parts per million, can also be used to describe solution concentrations.