The Mole

What is a mole? 1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of C-12.

The Mole 1 dozen = 12 1 gross = 144 1 ream = 500 1 mole = 6.02 x 1023 There are exactly 12 grams of carbon-12 in one mole of carbon-12.

Avogadro’s number and the mole – periodic table All the elements described on the periodic table has the mass number equivalent to the quantity of 1 mol Carbon-12 is quite significant – all other elements are standardised against carbon-12.

Molar mass The mass of one mole of any substance is known as its molar mass (symbol M), where the molar mass is equal to the relative atomic (or molecular or formula) mass measured in units of grams per mole (g mol–1).

Calculating RMM The relative molecular mass of a molecule is therefore the sum of the relative atomic masses of the constituent elements, as given in the molecular formula. The symbol for RMM is Mr. Worked example 1 Determine the relative molecular mass of the following molecules. a Water (H2O) b Methane (CH4) c Sulfuric acid (H2SO4)

Calculating RMM Worked example 1 Determine the relative molecular mass of the following molecules. a Water (H2O) b Methane (CH4) c Sulfuric acid (H2SO4) Solution a Mr(H2O) = 2 1.0 + 16.0 = 18.0 b Mr(CH4) = 12.0 + 4 1.0 = 16.0 c Mr(H2SO4) = 2 X1.0 + 32.1 + 4 X 16.0 = 98.1 Solution a Mr(H2O) = 2 1.0 + 16.0 = 18.0 b Mr(CH4) = 12.0 + 4 1.0 = 16.0 c Mr(H2SO4) = 2 1.0 + 32.1 + 4 16.0 = 98.1

Calculating RFM Worked example 2 Determine the relative formula mass of the following ionic compounds. a Sodium chloride (NaCl) b Magnesium hydroxide (Mg(OH)2) c Aluminium carbonate (Al2(CO3)3) Solution a RFM(NaCl) = 23.0 + 35.5 = 58.5 b RFM(Mg(OH)2) = 24.3 + (16.0 + 1.0) 2 = 58.3 c RFM(Al2(CO3)3) = 2 27.0 + (12.0 + 3 16.0) 3 = 234 The technique employed to calculate relative formula mass (RFM) is identical to that used for relative molecular mass. The only difference is that RFM is used for ionic compounds, which cannot be described as molecules.

Avogadro’s number is called “Avogadro’s Number” Avogadro’s number describes how many atoms are there within a mole. There is Avogadro’s number of particles in a mole of any substance.

Mass and mole Because of the relationship of the mole, we can write that n = number of moles m = mass (g) M = molar mass ( g )

Sample Question How many atoms are there within 1.5 mol of carbon?

Calculating number of particles How many atoms are there within 1.5 mol of carbon?

Sample Question How many moles is atoms worth?

Changing mass to mol In a 6 g solution of pure carbon, how many carbon atoms does it contain?

Molar mass The molecular mass (m) of a substance is the mass of one molecule of that substance.

Changing mol to mass With a solution that has 2 mol of lithium, where the molecular mass of lithium is 7, find the actual mass of the solution

Practise Give the molar masses of the following compounds: 1. sodium fluoride -NaCl 2. potassium hydroxide -KOH

Answer (1 x 23 ) + (1 x 19 ) = 42 grams/mole of sodium fluoride 2. (1 x 39) + (1 x 1) + (1 x 16) = 56 grams/mole of potassium hydroxide