The Mole Avogadro’s Number

“The Mole” is simply the name given to a certain number It is a unit of measurement, just as the word ‘dozen’ represents 12 and ‘kilo’ represents 1000. But the mole represents a much bigger number. To represent just how big a mole is, we can compare the following: ‘a million’ is this many: 1 ‘a billion’ is this many: 1 ‘a trillion’ is this many: 1

‘A Mole’ is this many: 6.203 x 10 23 602 3 or simply or simply 6.203 x 10 23 Also know as Avogadro’s Number The number of atoms in one mole of an element is 6.02 x 10 23

So what does the Mole do? The Mole gives us a way in which we can count atoms and molecules. When you get precisely that number of atoms or molecules, of any element or compound, then conveniently, they weigh exactly the same number of grams as the Relative Atomic Mass, 𝐴 𝑟 (or 𝑀 𝑟 ) of the element or compound. ONE MOLE of atoms or molecules of any substances will have a mass in grams equal to the Relative Formula Mass ( 𝐴 𝑟 or 𝑀 𝑟 ) for that substance.

Molar Mass (M) The molar mass (M) of an element is defined as the mass of 1 mol of the element. The unit is grams per mole (g mol–1). For example: Molar mass of carbon atoms = mass of 1 mol of C atoms = 12 g 𝑚𝑜𝑙 −1 ∴ 12 g of carbon contains 6.02 x 10 23 atoms of carbon. Molar mass of oxygen atoms = mass of 1 mol of O atoms = 16 g 𝑚𝑜𝑙 −1 ∴ 16 g of oxygen contains 6.02 x 10 23 atoms of oxygen.

Your turn: Carbon has an 𝐴 𝑟 of 12. So one mole of carbon weighs exactly 12g Iron has an 𝐴 𝑟 of 56. So one mole of iron weighs exactly 56g Nitrogen gas, 𝑁 2 , has an 𝑀 𝑟 of 28. So one mole of 𝑁 2 weighs exactly 28g Carbon dioxide, 𝐶𝑂 2 , has an 𝑀 𝑟 of 44. So one mole of 𝐶𝑂 2 weighs exactly 44g This means that 12g of carbon, or 56g of iron, or 28g of 𝑁 2 , or 44g of 𝐶𝑂 2 , all contain the same number of atoms, namely ONE MOLE or 6.203 x 10 23 atoms or molecules.

Compounds and Molar Mass The molar mass of a compound is the mass of one mole of that compound. It is found by adding together all the relative atomic masses for the atoms in its formula and adding ‘g 𝑚𝑜𝑙 −1 ’ as the unit. An example of a molecular compound is: Molar mass of water molecules = mass of 1 mol of 𝐻 2 O molecules = 18 g 𝑚𝑜𝑙 −1 ∴ 18 g of water contains 6.02 x 10 23 molecules of water. How is it 18 g 𝑚𝑜𝑙 −1 ? Atomic Mass of Oxygen is: 16 Atomic Mass of Hydrogen is: 1 So, One Oxygen molecule + Two Hydrogen molecules = 16 molecules

Ionic Compounds The 𝑀 𝑟 is found by adding together the 𝐴 𝑟 of each atom in the formula of the compound. For example: 𝑀 𝑟 of 𝐶𝑢𝑆𝑂 4 = 𝐴 𝑟 (Cu) + 𝐴 𝑟 (S) + 4 x 𝐴 𝑟 (O) = 63.5 + 32.1 + 4 x 16.0 = 159 6 Molar mass of 𝐶𝑢𝑆𝑂 4 = 159.6 g 𝑚𝑜𝑙 −1