Finding the formula of Magnesium Oxide by experiment

A Piece of magnesium was heated strongly. The formula of magnesium oxide can be found by massing.

Mass of crucible + Lid = 17.14g
Mass of crucible + Lid + Magnesium = 17.32g
Mass Magnesium = 0.18g
Mass of crucible + Lid + Magnesium Oxide = 17.42g
Mass of crucible + Lid + Magnesium = 17.32g
Mass of Oxide = 0.10g

Mg = 0.18 O = 0.10
MMM24 MMO16

= 0.0075 MM= 0.00625

= 1.2 : 1

We see there is too much Magnesium.

Reasons for Error

• Not all the Mg was burned.
• Some product escaped as smoke when the lid was lifted.
• Hot Magnesium will react with Nitrogen in the air if there is insufficient O₂ forming Magnesium Nitrate (Mg₃N₂).

Using Equations to find reacting Masses

Example :

Iron reacts with chlorine according to equation

2Fe + 3CL₂ à 2FeCl₃

Questions :

1. What is the M_r of Iron III chloride?
2. What mass of Iron would be needed to make 16.25 tonnes of Iron III chloride

Answers :

1. M_r (FeCl₃) = 56 + (3x35.5) = 162.5
2. 2Fe + 3Cl à Fe₂Cl₃

2moles + 3moles IIà 2moles
2x56 MMMMMMà 2x162.5
56g MMMMMMIà 162.5g
5.6g MMMMMMà 16.25g
5.6 tonnes MMMIà 16.25 tonnes

A coal fired power station burns 1000 tonnes of coal a day. Calculate the mass of sulphur dioxide released in a day if the coal contains 1% sulphur

Mass of S burnt in one day = 0.01 x 1000 = 10 tonnes

S + O à SO₂
1mole MMà 1mole
32g MMMà 64g
1g MMMIà 2g
10g MMNà 20g
10 tonnes à 20 tonnes

Gas Molar volume

One mole of ANY gas at room temperature and pressure occupies a volume of 24 litres

Examples

Volume occupied by 3 moles = 3 x 24 = 72 litres (72dm³)
0.02 moles H_2(g) = 0.02 x 24 = 0.48 litres
8.8g CO2 = ^8.8/₄₄ = 0.2 moles CO₂

which occupies 0.2 x 24 = 4.8 litres