IGCSE & A/AS Levels students Forecast

AS & A Levels: Quantitative Chemistry part 1

THE MOLE

A mole of a substance is the amount of that substance that contains the same number of elementary particles as there are carbon atoms in 12.00000 grams of carbon-12.

One mole of carbon-12 has a mass of 12.0g.

One mole of hydrogen atoms has a mass of 1.0g.

One mole of hydrogen molecules has a mass of 2.0g.

One mole of sodium chloride has a mass of 58.5g.

The number of particles in one mole of a substance is 6.02 x 10²³. This is known as Avogadro's number, L.

 Thus when we need to know the number of particles of a substance, we usually count the number of moles. It is much easier than counting the number of particles.

 

The number of particles can be calculated by multiplying the number of moles by Avogadro’s number. The number of moles can be calculated by dividing the number of particles by Avogadro’s number.

The mass of one mole of a substance is known as its molar mass, and has units of gmol^-1. It must be distinguished from relative atomic/molecular/formula mass, which is a ratio and hence has no units, although both have the same numerical value.

The symbol for molar mass of compounds or molecular elements is Mr. The symbol for molar mass of atoms is Ar.

Mass (m), molar mass (Mr or Ar) and number of moles (n) are thus related by the following equation:

Mass must be measured in grams and molar mass in gmol^-1.

REACTING MASSES

It is possible to use the relationship moles = mass/m_r to deduce the masses of reactants and products that will react with each other.

When performing calculations involving reacting masses, there are two main points which must be taken into account:

The total combined mass of the reactants must be the same as the total combined mass of the products. This is known as the law of conservation of mass.

The ratio in which species react corresponds to the number of moles, and not their mass. Masses must therefore all be converted into moles, then compared to each other, then converted back.

i)  reactions which go to completion

Eg What mass of aluminium will be needed to react with 10 g of CuO, and what mass of Al₂O₃ will be produced?

3CuO(s) + 2Al(s) à Al₂O₃(s) + 3Cu(s)

       10 g

= 10/79.5

= 0.126 moles of CuO

3:2 ratio with Al

so 2/3 x 0.126 = 0.0839 moles of Al, so mass of Al = 0.0839 x 27 = 2.3 g

3:1 ratio with Al₂O₃

so 1/3 x 0.126 = 0.0419 moles of Al₂O₃, so mass of Al₂O₃ = 0.0419 x 102 = 4.3 g