4S1Tuesday, 1/3/11

10.45 – 11.55 am

Instead of writing long description of

chemicals or substances,

chemists write their chemical formulae.

For example, hydrogen gas is

written as H2

So, what is a chemical formula???

Chemical formula= a representation of a chemical substance using letters for atoms & subscript numbers to show the numbers of each type of atoms that are present in the substance

N2

The letter ‘N’ shows the symbol of nitrogen atom

The subscript ‘2’ shows that there are 2 nitrogen atoms in a molecule of

nitrogen gas

Li2OThe subscript ‘2’ shows that there are 2 lithium atoms in a molecule of

lithium oxide, Li2O

The letter

show the symbols of the

element

There is no

subscript here. This

means that there is only 1 atom of

oxygen in this

molecule

Empirical formula = the simplest whole

number ratio of atoms of each element in the

compounde.g.: formula of glucose = C6H12O6

E.F of glucose = CH2O

Determining empirical formula of

a compound

Refer page 42 of Textbook

Element Ca FMass of

element, m (g)

0.20 0.39 – 0.20 = 0.19

No. of moles, n

(mol)

0.2040

= 0.005

0.1919

= 0.01Ratio of moles

0.0050.005= 1

0.010.005= 2

E. F. = CaF2

Element Na S O

Refer page 42 of Textbook

% 32.4 22.6 45.0

Mass, m (g) 32.4 22.6 45.0

No. of moles, n

(mol)

32.423=

1.409

22.632

= 0.706

45.016

= 2.813

Ratio of moles

1.4090.706= 2

0.7060.706= 1

2.8130.706= 4

E. F. = Na2SO4

Try Q3 of Work This

Out 3.7 Calculation

page 42

Refer page 42 of Textbook

Element Al SMass of element,

m (g)60 – 38.4

= 21.638.4

No. of moles, n (mol)

21.627

= 0.8

38.432

= 1.2Ratio of moles 0.8

0.8= 1

1.20.8

= 1.5

E. F. = Al2S3

Simplest ratio of moles

2 3

If empirical formula is the simplest

whole number ratio, what is molecular

formula???

Molecular formula= the actual number

of atoms of each element that is

present in a molecule

Molecular formula = (Empirical formula)n

Compound Empirical formula

Molecular formula

n

Water H2O (H2O)1 = H2O

1

Ethene CH2 (CH2)2 = C2H4

2

Glucose CH2O (CH2O)6 = C6H12O6

6

Determining molecular

formula of a compound

Tips: must know RMM & E.F. of the cpd to

know its M.F.

Refer page 43 of Textbook

E.F. = CH2, RMM = 42. M.F.? [RAM: H=1, C=12]

Assume M.F. = (CH2)n

Based on the M.F., RMM = n [12 + 2(1)] = 14n

However, RMM = 42 14n = 42 n = 42 = 3

14 M.F. = (CH2)3 = C3H6

Refer page 44 of Textbook

E.F. = C2H5, RMM = 58. M.F.? [RAM: H=1, C=12]

Assume M.F. = (C2H5)n

Based on the M.F., RMM = n [2(12) + 5(1)]

= 29nHowever, RMM = 58 29n = 58 n = 58 = 2

29 M.F. = (C2H5)2 = C4H10

Try Q2 of Work This

Out 3.8 Calculation

page 44

Refer page 44 of Textbook

E.F. = CH2O, RMM = 60. M.F.? [RAM: H=1, C=12]

Assume M.F. = (CH2O)n

Based on the M.F., RMM = n [12 + 2(1) + 16]

= 30n However, RMM = 60 30n = 60 n = 60 = 2

30 M.F. = (CH2O)2 = C2H4O2

REMINDER!!!Memorise the ionic formulae of cations and anions for the next lesson