1.25 calculate reacting masses using experimental data and chemical equations

This means knowing that the mass of A and the mass of B will equal the mass of AB.
Bearing in mind that the mass of one mole of an element is its atomic mass.
If we have 2 moles of A(24) and react it with 5 moles of B(25)
then we have (2x24)+(5x25) and can calculate that A2B5 will weigh 173g

1.24 calculate empirical and molecular formulae from experimental data

Find the masses of the elements in the compound (by weight the compound then weighing it with elements taken out). Make these masses into percentages, divide each percentage by the ar of that element and you will have the number of atoms of it in a molecule.

e.g
Carbon= % mass 80, ar 12
C= 80/12= 6.7
Hydrogen= % mass 20, ar 1
H= 20/1= 20
C6.7H20

To find the empirical formula divide the molecular formula by the smallest number.

e.g
C6.7H20 /6.7
=CH3

1.23 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation

Weigh you compound, remove one element of the compound through a reaction (break up a metal oxide or salts), then weigh again.

The first weight is AB the second weight is A so doing AB-A= B.

Now that you have the weight of A and B you can work out the formulae by doing the weight divided by the Ar.

e.g

A= 22g with an ar of 11

A= 22/11= 2

B=72g with an ar of 18

B=72/18= 4

A2B4 and to simplify to empirical formula AB2

1.22 use the state symbols (s), (l), (g) and (aq) in chemical equations to represent solids, liquids, gases and aqueous solutions respectively

(S) solid
(L) liquid
(G) gas
(Aq) aqueous/ solid dissolved in liquid

In balanced equations these symbols go after a element or compound to show what state it is in.

1.21 write word equations and balanced chemical equations to represent the reactions studied in this specification

Word equations have just the names of the reactants and products involved:
Hydrogen + Oxygen > Water
Balanced equations are the symbols of the products and reactants including the numbers of each, there must be an equal number of each element on both sides of the equation, if there are not you can alter this by putting the right number infront of a symbol:
2H + O > H2O

1.19 carry out mole calculations using relative atomic mass (Ar) and relative formula mass (Mr)

The way I like to do this is to have a triangle with Mass (g) on top, with Moles (mol) bottom left and Mr/Ar bottom right.

1.18 understand the term mole as the Avogadro number of particles (atoms, molecules, formulae, ions or electrons) in a substance

A mole is Avogadro's number simpy because if you have 1 mole of an element its weight in grams will be its atomic mass.

1.17 understand the use of the term mole to represent the amount of substance

A mole is an amount, in the same way that you can have a dozen buns, you can have a mole of carbon.
Having a mole of something is having 6.022x1023 of it.

n.b the 1023 thing is meant to be 10 to the power of 23.

1.16 calculate relative formula masses (Mr) from relative atomic masses (Ar)

Mr is relative formula mass, it is the mass of a molecule. Its calculation is (number of that element in the molecule x relative atomic mass) + (number of that element in the molecule x relative atomic mass)

Ar is relative atomic mass, it is the average mass of an atom of a specific element. It is calculated by doing (% of isotope x mass of isotope) + (% of isotope x mass of isotope) over 100.