4s1 tuesday, 1/3/11 10.45 – 11.55 am. instead of writing long description of chemicals or...
TRANSCRIPT
Instead of writing long description of
chemicals or substances,
chemists write their chemical formulae.
For example, hydrogen gas is
written as H2
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
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
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
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
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
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