Empirical and Molecular Formulae

7/29/2019 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

Empirical and Molecular Formulae

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