The Mole

• Revision from S.G.• 1 mole is the formula mass in grams.• Concentration of solutions is measured in mol/l.• We use moles to Balance equations.• Example – Calculate the mass of 0.5 moles of

Water.

• Formula mass = H2O = 2 + 16 = 18

• 1 mole = 18 g, 0.5 moles = 0.5 x 18g = 9g

Avagadro’s Constant (L)

1 mole of any substance contains the same amount of “ elementary entities” as 12g ( 1 mole) of C – 12.

“ Elementary entities” can be; atoms, molecules, ions, units, protons, electrons etc.

Avagadro’s constant = 6.02 x 10 23 mol/l.

Example

• Water – H2O• How many molecules are in 1 mole – 18g- of

water?• 1 x L = 6.02 x 10 23 molecules.• How many atoms in 1 mole – 18g – of water?• Each molecule is made of 2 H and 1 O atoms – a

total of 3 atoms.• 1 mole of water contains 3 x L = 3 x 6.02 x 10 23.

= 1.8 x 10 24 atoms

Water cnt.

• How many molecules are there in 0.5 moles of water?

• 1 mole = 1 x L• 0.5 moles = 1.5 x l x L = 0.5 x1 x 6.02 x 10 23.

» = 3.01 x 10 23 molecules

• How many atoms are there in 0.1 moles of water?• In 1 mole there are 3 x L atoms.• In 0.1 moles – 0.1 x 3 x L • = 0.1 x 3 x 6.02 x 10 23 = 1.8 x 10 23 atoms

More examples

• How many protons are there in 1 mole of carbon?• In 1 atoms of Carbon there are 6 protons( atomic

number)• In 1 mole there will be 6 x L protons

= 6 x 6.02 x 10 23 = 3.6 x 10 24 protons.How many protons are there in 0.1 moles of Carbon?1 mole = 6 x L0.1 moles = 0.1 x 6 x L0.1 x 6 x 6.02 x 10 23 = 3.6 x 10 23 protons.

Molar Volume

• The average Molar Volume of a gas can be taken as 22.4 l – at the same standard temperature and atmospheric pressure.

• We can calculate molar volume using the following:

MD V

M = mass , D = Density, V = Volume

Worked Examples!

• Data:

• Mass of empty flask = 107.49g

• Mass of flask + Carbon dioxide = 108.37g

• Volume of flask = 480 cm3.

• Calculate – Mass of CO2 = 108.37 – 107.49= 0.88g.

• 0.88g of CO2 has a volume 0f 480 cm3.

The Answer!

• What about 1 mole?

• I mole of CO2 = 44g

• 44g has a volume of: 44/0.88 x 480=

24,000 cm3

The molar volume of Carbon dioxide is 24,000 cm3 or 22.4 litres.

Calculating Density

• Example

• Calculate the density in g /l of Neon, molar volume – 24l.

• D = M/V

• D = 20.2/24

• D = 0.84 g/l

Density cnt

• Calculate the density of Ammonia gas in g/cm3, molar volume 24 l.

• D = M/V

• D = 17/24

• D =0.71g /l or 0.00071g / cm3

• Calculate the molar volume of Argon if it has a density of 0.0018g cm3

• V = M/D

• V = 40/0.0018

• V = 22, 222cm3 or 22.2 l

Calculations using gas volumes

• Calculate the volume and composition of the residual gas mixture when 20 cm3 of methane is burned completely in 100cm3 of Oxygen.

• CH4 + 2O2 —> CO2 + 2H2O• 1 mole : 2 moles 1 mole : 2 moles• 20 cm3: 40 cm3 20 cm3: 40 cm3

• The residual gas mixture will contain:• 20 cm3 CO2, 40 cm3 H2O and 60 cm3 un reacted

oxygen.( The Oxygen is in excess!)