The mole: The concept of mole (symbol mol) has been accepted and adopted as an International System of Unit (S.I) for the amount of substance of a system which contain as many elementary entities as there are atoms in 0.012 kilograms of carbon - 12.
A mole of any substance contains Avogadro's number of molecules, atoms, ions or electrons. A dozen eggs is equal to 12 eggs. Similarly, a mole of ethanol equals 6.02 x 10^23 ethanol molecules.
In using this term mole for ionic substances, it is meant the number of formal units of that substance. For example, a mole of sodium carbonate, Na2Co3, is a quantity containing 6.02 x 10^23 Na2CO3 units. However, each formal unit of Na2CO3 contains two Na+ ions and one CO3- ion. Therefore, a mole of Na2CO3 also contains 2 x 6.02 x 10^23 Na+ ions and 1 x 6.02 x 10^23 CO3- ions.
When using the term mole, it is important to specify the units or entities to avoid any confusion. For example, a mole of oxygen atoms contains 6.02 x 10^23 atoms. A mole of oxygen molecules contains 6.02 x 10^23 oxygen molecules.
The molar mass:
The molar mass of a substance is the mass of one mole of that substance. Carbon-12 has a molar mass of 12g / mol, by this definition. For all substances, the molar mass in grams per mole is numerically equal to the molecular weight in atomic mass units (amu). For example, ethanol, whose molecular formula is C2H5OH, has molecular weight of 46.1 amu and therefore a molar mass of 46.1 g / mol.
The concepts of mole and molar mass are useful in calculating the mass of an atom or a molecule. Some times, it is required to find the mass of one mole of a substance or to know how much does a given number of moles of a substance weigh? These can be explained through few numerical examples to make distinctions clear.
For example, if one wants to know the mass in grams of a chlorine atom, he can do so in few steps. The atomic weight of chlorine Cl is 35.5 amu, so its molar mass is 35.5 g / mole. Since a mole of any substance contains Avogadro's number of atoms, the number of atoms in 35.5 grams of chlorine are 6.02 x 10^23. Therefore, the mass of one chlorine atom is given by,
35.5 / (6.02 x 10^23) = 5.9 x 10^-23g.
The molar mass concept can be used to convert gram mass of a substance into moles. Suppose one is interested to know how many moles are present in a 10g sample of ethanol, C2H5OH. It is known from the molecular weight that 1 mol of ethanol weighs 46.1 g, or 1 g contains 1 / 46.1 mol. Therefore, 10g of ethanol contains 10 x 1/ 46.1 = 0.27 moles.