3E Chemical Formulae

8/13/2019 3E Chemical Formulae

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Chemical Formulae

A Chemical formulais a representation of achemical substance using letters for atoms andsubscript number to show the number of eachtype of atoms in the substance.

International Union of Pure and Applied

Chemistry (IUPAC)standardized the usage ofname, symbol, chemical formulae and mass ofatom or molecule for chemical substances atinternational level.


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Chemical Formula of an Element

The chemical formula of an element represents its atoms.

For some elements which naturally exist as molecules, thechemical formulae represent their molecules

N2Shows that there are 2atom of nitrogen in a

molecule of nitrogen gas, N2

Symbol ofnitrogen

atom


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Chemical Formulae of

A Compound

CO2

symbol of carbon and oxygen atoms

Shows

number of

oxygen atoms

No number means there

is only 1 carbon atom in

a molecule of carbon

dioxide


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A Compound

Ca(NO3)2

symbol of calcium, nitrogen and oxygen atoms

The bracketsshow that there

are 2 group of

nitrate ions


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Empir ical Formula

EmpiricalFormulaof a compound is the

simplest whole number ratio of atoms of

each element in the compound.

Example :chemicalformula of glucose molecule is C6H12O6Ratio of Element C : H : O

Ratio of atoms 6 : 12 : 6

Simplest ratio of atoms 1 : 2 : 1

Empirical Formula of Glucose CH2O


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Empir ical Formula

CompoundMolecular

Fomula

Empirical

Formula

water H2O H2O

methane CH4 CH4

Benzene C6H6 CH

Hydrogen peroxide H2O2 HO


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Calculation Problem of EF

Example 1:

In an experiment, hydrogen gas react with 4.53g

of hot metal X oxide. At the end of experiment,3.57g of metal X is obtained. What is the

empirical formula for the metal X oxide?

[ Relative atomic mass : O, 16 ; X,119 ]


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Solution of Example 1

Element X O

Mass ofelement /g3.57 4.53-3.57=0.96

Number of

Moles

Simplest ratio

of moles

Empirical

Formula

XO2

030119

573 060

16

960

1030

030

2030

060


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Calculation Problem of EF

Example 2:

A compound consist 41.05% of potassium, K;

33.69% of sulphur, S and 25.26% of oxygen, O.

Find the empirical formula for this compound.

[ Relative atomic mass : K, 39 ; S, 32 ; O, 16 ]


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Solution of Example 2

Element K S O

Mass of element

in 100g/g41.05 33.69 25.26

Number of

moles

Simplest ratio

of moles

Ratio of moles

in integer2 2 3

Empirical

Formula K2S2O3

05139

0541 051

32

6933 581

16

2625

1051

051

1051

051

51051

581


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Molecular Formula

Molecular Formula of a compound is the actual

number of atoms of each element that are present

in a molecule of the compound.

In a Molecular Formula, the subscript number are

simple multiple of those in the empirical formula.

Molecular Formula = ( Empirical Formula )n


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Comparison of EF and MF

CompoundEmpirical

Formula

Molecular

Fomulan

Water H2O (H2O)1= H2O 1

methane CH4 (CH4)1=CH4 1

Benzene CH (CH)6= C6H6 6

Hydrogen peroxide HO (HO)2=H2O2 2


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Calculation Problem of MF

Example 1:

The empirical formula of a compound is

C2H4O. It has a relative molecular mass of

88. What is the molecular formula for this

compound?

[ Relative atomic mass: C,12 ; H, 1; O,16 ]


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Solution

Assume that the molecular formula is (C2H4O)n

Relative molecular mass of compound=88

88= [2(12) +4(1) +16 ]n

88=44n

n=2

Thus, Molecular Formula = ( C2H4O)2

= C4H8O2


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Percentage composition by mass of

an element in a compound

% composition by mass of an element in a compound

=%100

compoundmol1ofmass

compoundinelementofmass


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Problem of Percentage composition by

mass of an element in a compound

Find the percentage composition by mass of

nitrogen in urea, CO(NH2)2.

[AR: C,12 ; O, 16; H, 1 ; N,14 ]

% composition by mass of Nitrogen in urea

%67.46

%100)]1(214[21612

)14(2

%100ureaofmol1ofMass

ureaofmol1inatomsnitrogenofMass


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I onic Compound

CaCO3Symbol of calcium,

Ca; carbon, C and

oxygen, O atoms

Number 3

shows the

quantity of

oxygen in the

compound


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Chemical Formulae for

Ionic Compounds

Ionic compounds are made up ofpositively- charged ions (cations) and

negatively-charged ions (anions)

Ionic compounds are electricallyneutral

even though they contain charged particles.This is because the total of positive chargesare equal to the total of negative charges


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L ist of some common cations

Formula

of cationName of cation

Formula


H+ Hydrogen Ion Ca2+ Calcium ion

Li+ Lithium ion Mg2+ magnesium ion

Na+ Sodium ion Ba2+ Barium ion

K+ Potassium ion Zn2+ Zinc ion

Ag+ Silver ion Ni2+ nickel(II) ion

NH4+ Ammonium ion Al3+ Aluminium on

Co2+ Cobalt (II) ion Cr3+ Cromium(III) Ion


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L ist of some common cations

Formula


Formula


Cu+ Copper (I) ion Pb4+ Lead (IV) ion

Cu2+ Copper (II) ion Hg+ Mercury (I) ion

Fe2+ Iron (II) ion Hg2+ Mercury (II) ion

Fe3+ Iron (III) ion Mn2+ Manganese (II) ion

Sn2+ Tin (II) ion

Sn4+ Tin (IV) ion

Pb2+

Lead (II) ion


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L ist of some common anions

Formula

of anion

Name of

anion

Formula

of anionName of anion

F Fluoride ion OH Hydroxide ion

Cl Chloride ion ClO Chlorate (I) ion

Br Bromide ion ClO3 Chlorate (V) ion

I Iodide ion HCOO Methanoate ion

NO3 Nitrate ion CH3COO Ethanoate ion

NO2 Nitrite ion MnO4

Manganate(VII)

ion

H Hydride ion


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L ist of some common anions

Formula


Formula


BrO Bromate (I) ion SO42 Sulphate ion

BrO3 Bromate (V) ion S2O32

Thiosulphate ion

IO Iodate (I) ion CO32 Carbonate ion

IO3 iodate(V) ion CrO4

2 Chromate (VI) ion

O2 Oxide ion Cr2O72 Dichromate (VI) ion

S2 sulphide ion PO43 Phosphate ion

SO3 2 Sulphite ion


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CF of simple Ionic Compound

Example 1:

What is the chemical formula of Silver iodide?

Ions Ag+ I

Charge of ion +1 1

Number of ions 1 1

Chemical Formula of Silver iodide = AgI


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Example 2:

What is the chemical formula of zinc oxide?

Ion Zn2+

O2

Charge of ion +2 2

Number of ion 2 2

Simplest ratio of ions 1 1

Chemical formula of zinc oxide = ZnO


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Example 3:

What is the chemical formula ofIron (III) chloride?

Ion Fe3+ Cl

Charge of ion +3 1

Number of ion 1 3

Chemical formula of Iron (III) chloride = FeCl3


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Practice 1: Construct Chemical Formulae

Construct the chemical formula for these compounds:

1. Potassium chloride

2. Sodium bromide

3. Lithium iodide

4. Aluminium oxide

5. Copper (II) chloride

6. Calcium fluoride

7. Lead (II) bromide

8. Iron (II) sulphide

9. Barium chloride

10. Chromium (III) oxide


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CF of complicated Ionic Compound

Complicated ions are ions that made up oftwo or more elements such asNO3

, CO32,

SO42

, OH

etc.

Brackets are used to show the number ofthese ions involved in formulae

If there is only 1 ion involved, brackets isnot necessary. Example CuCO3


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Example 1:What is the chemical formula of ammonium sulphate ?

Ion NH4+ SO4

2

Charge of ion +1 2

Number of ions 2 1

Chemical Formula of ammonium sulphate

=(NH4)2SO4


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Example 2:What is the chemical formula of calcium carbonate?

Ion Ca2+ CO32

Charge of ion +2 2

Number of ions 2 2

Simplest ratio of ions 1 1

Chemical formula of calcium carbonate =CaCO3


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Example 3:What is the chemical formula of iron (III) nitrate?

ion Fe3+ NO3

Charge of ion +3 1

Number of ions 1 3

Chemical formula of iron (III) nitrate =Fe(NO3)3


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P ti 2 C t t h i l f l f


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Practice 2: Construct chemical formula of

complicated ions

1. Ammonium nitrate

2. Copper (II) nitrate

3. Calcium hydroxide

4. Sodium thiosulphate

5. Barium sulphate

6. Magnesium carbonate

7. Aluminium nitrate

8. Potassium ethanoate

9. Zinc sulphate

10. Iron (II) sulphite

11. Iron (II) sulphate

12. Tin (II) nitrate

13. Lead (II)

Chromate (VI)14. Barium dichromate (VI)

15. Ammonium phosphate

16. Chromium (III) hydroxide

17. Nickel(II) carbonate18. Potassium chlorate(V)

19. Sulphuric acid

20. Nitric acid

Construct chemical formula for these chemical

compound:


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Naming of Chemical Compounds

Chemical compound are named

systematically according to the

guidelines given by IUPAC

( International Union of Pure and

Applied Chemistry)


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Guideline to name a

Chemical Compound

For ionic compound

name of cation comes first, followed by the

name of anion

Cation AnionName of ionic

compound

Sodium ion Chloride ion Sodium chloride

Zinc ion Oxide ion Zinc oxide

Calcium ion Carbonate ion Calcium

carbonate


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Guideline to name a

Chemical Compound

For simple molecular compounds:

The name of first element is maintain but the nameof second element is added with an -ide

Examples: HCl - hydrogen chlorideKBr - potassium bromide

NaO - sodium oxide


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Guideline to name a

Chemical Compound

Greek prefixes are used to show the number of

atoms of each element in a compound.

Example: CO

carbon monoxideCO2carbon dioxide

SO3sulphur trioxide

CCl4 - carbon tetrachloride

PCl5

phosphorus pentachloride

N2O4Dinitrogen tetroxide

Cl2O7 - dichlorine heptoxide


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Meaning of Greek Pref ixes

Prefix Meaning Prefix Meaning

Mono 1 Hexa- 6

Di- 2 Hepta- 7

Tir- 3 Octa- 8

Tetra- 4 Nona- 9

Penta- 5 Deca- 10


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Solution of Practice 1

1. KCl

2. NaBr

3. LiI

4. Al2O3

5. CuCl2

6. CaF2

7. PbBr2

8. FeS

9. BaCl2

10. Cr2O3


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Solution of Practise 2:

1. NH4NO32. Cu(NO3) 2

3. Ca(OH)24. Na2S2O35. BaSO46. MgCO3

7. Al(NO3)38. CH3COOK

9. ZnSO410. FeSO3

11. FeSO412. Sn(NO3)2

13. PbCrO414. BaCr2O715. (NH4)3PO416. Cr(OH)3

17. NiCO318. KClO319. H2SO420. HNO3

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3E Chemical Formulae