Empirical and Molecular Formulae
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Transcript of Empirical and Molecular Formulae
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CHEMICAL
FORMULAE
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At the end of this lesson, students
should be able to:
State the meaning of chemical formulaeState the meaning of empirical formula
State the meaning of molecular formula
Determine the empirical and molecular formulae ofsubstances
Compare and contrast the empirical formula with
molecular formula
Solve numerical problems involving empirical andmolecular formulae
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Chemical Formulae
A representation of a chemical substance
using letters for atoms and subscript
numbers to show the numbers of each type
of atoms that are present in the substance.
Formula of water : HO
H is the symbol which represents hydrogen.
O is the symbol which represents oxygen.
The number 2 represents the number of hydrogenatoms present.
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CHO
(1) Empirical formula
CHO
(2) Molecular formula
2HCO
(3) Ionic formula
H O H
(4) Structural formula
FORMULA
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Empirical formula shows the simplest whole
ratio of atoms in a substance.
Molecular formula shows the actual numberof atoms in a substance.
Important!
The empirical formula might or might not be
the same as the actual molecular formula!
The empirical formula = molecular formula for
IONIC COMPOUNDSALWAYS!
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Dear 4S2,
Percent to mass
Mass to mole
Divide by smallMultiply til whole.
Sincerely,
Empirical formula
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The following is the percentage composition of calcium
carbonate:Ca=40%, C=12%, O=48%. Calculate the
empirical formula of calcium carbonate.
(Relative atomic mass:Ca,40; C,12; O,16)
Solution:
Element Calcium, Ca Carbon,C Oxygen, O
Mass of
element (g)
40 12 48
Number of
moles of
atoms
40
40= 1
12
12= 1
48
16= 3
Ratio of moles 1
1= 1
1
1= 1
3
1= 3
Simplest ratio
of moles
1 1 3Empirical formula:
CaCO
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1.44 g of magnesium combines chemically with
0.96 g of oxygen to form an oxide. What is the
empirical formula of the oxide?(Relative atomic mass: O,16; Mg,24)
Solution:Element Magnesium, Mg Oxygen, O
Mass of element
(g)
1.44 0.96
Number of moles
of atoms
1.44
24= 0.06
0.96
16= 0.06
Ratio of moles 0.06
0.06= 1
0.06
0.06= 1
Simplest ratio of
moles
1 1
Percent to massMass to mole
Divide by smallMultiply til wh ole.
Empirical formula:
MgO
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0.91 g of aluminium burns in air to form 1.7 g of aluminium
oxide. What is the formula of aluminium oxide?
(Relative atomic mass: Al,27; O,16)
Solution:
Element Aluminium, Al Oxygen, O
Mass of element(g) 0.91 1.7 0.91= 0.79
Number of moles
of atoms
0.91
27= 0.034
0.79
16= 0.05
Ratio of moles 0.034
0.034 = 1
.
.
= 1.5
Simplest ratio of
moles
1 x 2
= 2
1.5 x 2
= 3 Empirical formula:
AlO
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Relationship between empirical and molecular
formula of a substance.
Whereby n is a positive integer.
Example:
Molecular formula = ( Empirical formula ) n
CompoundEmpirical
formula
Molecular
formulan
Water HO(HO) = HO
1
Ethene CH(CH) = CH
2
Glucose CHO
(CHO) =
CHO 6
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Example:
The decomposition of 7.36 g of a compound produces 6.93 g of
oxygen. The rest of the mass is hydrogen. If the relative
molecular mass of this compound is 34.0 g, calculate its
molecular formula.(Relative atomic mass: H,1; O,16)
Solution:
Element Hydrogen,
H
Oxygen, O
Mass of element(g) 0.43 6.93
Number of moles of
atoms
0.43
1
= 0.43
6.93
16= 0.43
Ratio of moles 0.43
0.43= 1
0.43
0.43= 1
Simplest ratio of
moles
1 1
Empirical formula: HO
Molecular formula =(empirical formula)n
34 = (HO) n
34 = (1 + 17 ) n
n = 34 / 17
n = 2
Thus, molecular formula
is HO
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RM 100 RM 200 RM 300
RM 400 RM 500 RM 600 RM 700
RM 8OO RM 1000 RM 1500 RM 3000
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What is the meaning of empirical formulae.
A) Formula that shows the actual number ofatoms of each element in the compound.
B) Formula that shows the simplest ratio ofatoms of each elements are bonded together
C) Formula that shows how the atoms of each
elements are bonded together.D) Formula that shows the type of element in
the compound.
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Which of the following is the empirical formula
of CHO?
A) CHO
B) CHO
C) CHO
D) CHO
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Which compound has the empirical formula of
CHO?
A) HCOOH
B) CHCOOH
C) CHCOO CH
D) HCOOCH
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The diagram shows the molecular formula for
glucose. CHO
Which of the following substance has similar
empirical formula as that of glucose?
Substance Molecular Formula
A Ethanol CHO
B Ethylene oxide CHO
C Ethanoic acid CHO
D Methyl ethanoate CHO
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In an experiment, 24 g of element X react with
32 g sulphur to form a compound. What is the
empirical formula of that compound?
[RAM: X=6; S=32]
A) XS
B) XS
C) XS
D) XS
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A sample of iron oxide contains 5.6 g of iron and
2.4 g of oxygen, what is the empirical formula of
this compound?
A) FeO
B) FeO
C) FeO
D) FeO
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0.32 g of metal X combines with 0.36 g of non-
metal Y. What is the empirical formula for this
compound?[RAM: X=64, Y=35.5]
A) XY
B) XY
C) XY
D) XY
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The table shows the mass of elements M and O
in an oxide, and the relative atomic mass of
element M and O.
The empirical formula for oxide of M is
A) MO
B) MO
C) MO
D) MO
Element M O
Mass/ g 5.6 2.4
Relative atomic mass 56 16
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The table shows the mass of elements M and O inan oxide, and the relative atomic mass of element
M and O.
Which of the following formulae is the empiricalformula for the oxide of M?
A) MO
B) MO
C) MO
D) MO
Element M O
Mass / g 1.62 1.44
Relative atomic mass 27 16
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11 g oxide M reduced to form 6.2 g metal M.
Empirical formulae of oxide M is.
[RAM: O=16; M=31]
A) MO
B) MO
C) MOD) MO
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Stainless steel used to make surgical scalpelscontains 71.5% iron, 18.0% chromium, 8.0% nickel,and 2.5% other elements.
If the mass of surgical scalpels is 60.0g, what is themass of iron in it?
A) 4.8 g
B) 10.8 g
C) 42.9 g
D) 71.5 g