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 =1 gross =

1 ream =

1 mole =

12

144

500

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 1Determine the relative molecular mass of the following molecules.a Water (H2O)b Methane (CH4)c Sulfuric acid (H2SO4)

Calculating RMM

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

Calculating RFM

Worked example 2Determine 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)Solutiona RFM(NaCl) = 23.0 + 35.5 = 58.5b RFM(Mg(OH)2) = 24.3 + (16.0 + 1.0) 2 = 58.3c RFM(Al2(CO3)3) = 2 27.0 + (12.0 + 3 16.0) 3 = 234

Avogadro’s number

is called “Avogadro’s Number”

Avogadro’s number describes how many atoms are there within a mole.

2310022.6

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 )

M

mn

1mol

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?

2323 10033.910022.65.1

Sample Question

How many moles is atoms worth?

2410045.9

mol02.1510022.610045.9 2324

Changing mass to mol

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

molM

mn 5.0

12

6

2323 1011.310022.65.05.0 mol

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

gnMm 1472

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