C2aL13 Moles
Key Words
Atom - smallest particle into which an element can be chopped.
Balanced Symbol Equation - equation that describes a reaction, written in symbols rather than words, with the correct numbers of molecules.
Elements - substance consisting of one kind of atom only.
Grams - 1 g = 1/1000 kg.
Mole - the same number of atoms as there are in 12 grams of carbon-12.
Relative Formula Mass - mass of a compound worked out from the atomic masses of its elements.
Grade E
Atoms are tiny, so we don't bother counting the individual atoms in a crystal. It's like buying 1 kilo of sugar, rather than a million crystals. Chemists often use a standard amount, the mole. The mole is defined as:
the same number of particles as in 12 grams of carbon 12.
This number is massive, 6 × 1023.
The mass of a mole of any substance is the relative formula mass, expressed in grams. So a mole of potassium permanganate, KMnO4 will have a mass of:
Mass of 1 mole = 39 g + 55 g + (16 × 4) g = 158 g
When hydrogen burns to form water, we can write a balanced symbol equation:
2H2 + O2 ® 2H2O
Two moles of hydrogen react with 1 mole of oxygen to form two moles of water.
Grade C
We can extend this further to convert masses into moles:
Number of moles = mass of a substance
mass of one mole of the substance
Consider the reaction of magnesium and oxygen:
2Mg + O2 ® 2MgO
Suppose we have 6 g of magnesium (relative atomic mass = 24). We will have 6 ÷ 24 = 0.25 moles
So we can say how many moles are reacting in the equation:
0.25 moles Mg + 0.125 moles O2 ® 0.25 moles MgO
Oxygen molecules have a relative molecular mass of 32. So 6 g magnesium react with:
0.125 × 32 = 4 g of oxygen molecules
This gives us 0.25 moles of MgO (which has a relative mass of 24 + 16 = 40). Therefore the mass of 0,25 moles MgO:
0.25 × 40 = 10 g MgO
We can check this since mass must be conserved:
6 g Mg + 4 g O2 ® 10 g MgO
Grade A
It is worth practising mole calculations. They are important in analytical chemistry.