Introduction to Chemistry

By: Mahmoud Taha

Special thanks to Ms Williams and Ms Matrella for their constant

support and inspiration

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Please note that these guides are a collation of my personal notes, teachers' notes, chemistry

books, and websites such as chemguide, chemsheets, chemwiki and wikipedia.

Balancing Equations You can balance an equation by inspection but some are very hard to solve that way.

Consider this equation, it looks impossible to solve by inspection, so we use algebra.

Place a letter as a coefficient beside each substance in the equation.

The equate the elements coefficients:

Sodium balance:

Chlorine balance:

Sulphur balance:

Oxygen balance:

Hydrogen balance:

Set x as 1, so the rest will be: x=1, y=1/2 , z=1/2 , w=1/4 , u=1/2 and w=1

Now multiply everything by 4 to get integers: x= 4 , y=2 , z=2 , w=1 , u=2 and w=4

Hence,

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Valency / oxidation states

Elements Element Oxidation State

GROUP 1 +1

GROUP 2 +2

GROUP 3 +3

F -1

Cl -1 (Except in ClF and Cl2O)

O -2 (Except in F2O and H2O2)

H +1

H in Metal Hydride -1

Polyatomic Ions :

Name Formula (without charge) Charge

Phosphate PO4 -3

Sulphate SO4 -2

Carbonate CO3 -2

Dichromate Cr2O7 -2

Hydroxide OH -1

Nitrate NO3 -1

Nitrite NO2 -1

Hydrogen carbonate

(bicarbonate)

HCO3 -1

Ammonium NH4 +1

Solubility Rules The Salt Soluble Insoluble

Na, K & Ammonium salts

All Nitrates

ALL NO EXCEPTIONS

Sulphates MOST Pb/Ba/Ca

Chlorides MOST Pb/Ag

Oxides/Hydroxides/Carbonates Na/K/

Ba2+, Sr2+, Ca2+ are considered

slightly soluble.

THE REST

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Ionic Equations

1.Start with a balanced molecular equation.

2.Break all compounds with (aq) beside them into their ions:

-Indicate the correct formula and charge of each ion

-Indicate the correct number of each ion

3.Write (aq) after each ion

4.Bring down all compounds with (s), (l), or (g) unchanged.

2 Na3PO4 (aq) + 3 CaCl2 (aq) --> 6 NaCl (aq) + Ca3(PO4)2 (s)

6 Na+ (aq) + 2 PO43- (aq) + 3 Ca2+ (aq) + 6 Cl- (aq) --> 6 Na+ (aq) + 6 Cl- (aq) + Ca3(PO4)2 (s)

TO MAKE IT NET IONIC:

1- Cross out the spectator ions that are present.

2-Write the "leftovers" as the net ionic equation.

A simple way to remember it, is to write out ions which were aqueous and changed into

Solid, Liquid or Gas...or just vice versa

6 Na+ (aq) + 2 PO43- (aq) + 3 Ca2+ (aq) + 6 Cl- (aq) --> 6 Na+ (aq)+ 6 Cl- (aq) + Ca3(PO4)2 (s)

FINALLY

2 PO43- (aq) + 3 Ca2+ (aq) --> Ca3(PO4)2 (s)

The following picture is from chemsheets.co.uk :

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Mole & Avogadro's Constant 1 mole is the mass of a substance that contains as many fundamental units as 12g of carbon-

12. One mole of anything has 6.02 x 1023 constituent particles. That huge number is

Avogadro's constant.

Mr (relative molecular mass) is the mass of one mole of the molecule in grams. We find out

the Mr of a compound by adding the Ar (relative atomic mass) of the elements present in that

compound.

Hence to find the number of moles in a given sample (say 10g of water) you use:

Mole = Mass of sample/Mr

Mr of H2O = 1+1+16 =18

Hence Moles of water in the sample = 10/18

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Introduction to Stoichiometry Calculations For ease of explanation I am going to use Amount of Substance Workbook from

chemsheets.co.uk

The tick means known and the question mark means unknown and the 3 dots in a triangle

means hence. Remember to convert your masses to grams.

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The rest of the booklet covers limiting reagents, percentage yields, atom economy, reacting

gas volumes, solution calculations, titrations, empirical & molecular formulas. The booklet

can be opened here.

If the hyperlink doesn't open:

http://www.chemsheets.co.uk/Chemsheets%20AS%20008%20(Amount%20of%20substance)

.pdf

If you have any queries please contact me at mahtah1@gmail.com