# Counting Atoms - Moles (IGCSE CHEMISTRY) slide presentation

CH. 9 COUNTING ATOM

KKCS International Program

Chemistry IGCSE

Mr. Markus

MOLE CONCEPT

• The mole is defined as the amount of substance that contains as many particles as the number of carbon atoms, in exactly 12 grams of carbon-12

• The number is equal to 6.0 x 10 23  particles (Avogadro’s number)

• The mole is abbreviated as mol

• The mole is a counting unit used in chemistry to deal with atoms, molecules and ions.

• Example:

1 mole of carbon atoms = 6.0 x 10 23  C atoms

1 mole of ammonia molecules = 6.0 x 10 23  NH3 molecule

1 mole of sodium ions = 6.0 x 10 23 Na+  ions

• To change the number of particles to the number of moles :

number of moles = number of particles

6.0 x 10 23

• To change the number of moles to the number of particles

Number of particles = (number of moles) X (6.0 x 10 23 )

EXERCISE

1. A sample of sodium contains 3.0 x 1020  Calculate the number of moles of sodium.

2. Determine the number of molecules in 0,5 moles of carbon dioxide.

3. How many H atoms are in 0.80 moles of hexane. C6H14?

4. How many NH4+ ions are in 1.5 moles of (NH4)3PO4?

5. How many molecules are there in 0.2 mole of sulphur dioxide, SO2? How many atoms are present?

MASS MOLAR

• The mass in gram of 1 mole of substance is caleed its molar mass

• The molar mass of any substance is always equal to its relative formula mas in gram, :

a. The mass of 1 mole of atom is equal to its relative atomic mass in grams/Ar

b. The mass of 1 molelecules is equal to its relative molecular mass in grams/Mr

c. The mass of 1 mole of units of ionic compound is wqual to its relative formula mass in grams

Ex. 1

Relative atomic mass of magnesium, Mg = 24

Hence, mass of 1 mole of Mg atoms = 24 g,

i.e. Molar mass Mg = 24 g/mol

Ex.2

Relative molecular mass of Ammonia,

NH3 = 14 + 3(1) = 17

Hence, mass of 1 mole of NH3 molecules = 17 g

i.e. Molar mass of NH3 = 17 g/mol

Ex.3

Relative formula mass of nitrate ion,

NO3- = 14 + 3(16) = 62

Hence, mass of 1 mole of NO3- ions = 62 g

i.e. Molar mass of No3- = 62 g/mol

• To convert moles to mass, use the following:

mass of substance =

(number of moles of substance) x molar mass of subst.

• Mass to moles

number of mole of substance

=       mass of substance

molar mass of substance

Example:

Calculate the mass of 2.50 moles of Al atoms

Solution:

Relative atomic mass of Al (Ar Al) = 27.0

Molar mass of Al = 27 g/mol

Mass of 2.50 moles of Al atoms

= (moles) x (molar mass)

= 2.50 mol x 27.0 g/mol

= 67.5 g

1. Calculate the mass of 1.12 moles of sulphur trioxide molecules.

2. Calculate the number of moles of molecus in 66 g of carbon dioxide.

3. Calculate the  number of moles Pb(CH3COO)2 in 250 g of lead(II) ethanoate

4. Calculate the number of moles of each element present in 1 mole of each of these:

a. Sucrose, C12H22O11

b. Calcium phosphate, Ca3(PO4)2

1. Calculate the mass of 1.12 moles of sulphur trioxide molecules.

Mr SO3 = 32+3(16) = 80 g/mol

Mass of SO3 = 1.12 mol x 80 g/mol

= 89.6 g

2. Calculate the number of moles of molecus in 66 g of carbon dioxide.

Ans:

Mr CO2 = 12 + 2(16) = 44 g/mol

Moles of CO2 = 66 g = 1.5 mol

44 g/mol

1. Calculate the  number of moles Pb(CH3COO)2 in 250 g of lead(II) ethanoate

Ans:

Mr Pb(CH3COO)2 = 207 + 4(12) + 6(1) + 4(16)

= 325 g/mol

Moles of Pb(CH3COO)2 = 250 g       =  0.769 mol

325 g/mol

4.  Calculate the number of moles of each element present in 1 mole of each of these:

a. Sucrose, C12H22O11

Ans: Carbon = 12 moles, Hydrogen = 22 moles, Oxygen = 11 moles

b. Calcium phosphate, Ca3(PO4)2

Ans: Calcium 3 moles, Posphate 2 moles, oxygen 8 moles

5. Calculate the number of moles of each element present in 100 g of each of the following:

a. (NH4)2Cr2O7

b. Na2CO3.10H2