1:26 calculate relative formula masses (including relative molecular masses) (Mᵣ) from relative atomic masses (Aᵣ)
Relative formula mass (Mr) is mass of a molecule or compound (on a scale compared to carbon-12).
It is calculated by adding up the relative atomic masses (Ar) of all the atoms present in the formula.
The relative formula mass (Mr) for water (H2O) is 18.
Water = H2O
Atoms present = (2 x H) + (1 x O)
Mr = (2 x 1) + (1 x 16) = 18
In Chemistry, the mole is a measure of amount of substance (n).
The abbreviation for mole is mol.
The mass of 1 mole of a substance is the relative formula mass (Mr) of the substance in grams.
The Mr of water is 18.
Therefore the mass of 1 mol of water equals 18 g.
This video steps through a very useful method used to calculate reacting masses.
3:03 calculate the heat energy change from a measured temperature change using the expression Q = mcΔT
Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.
EXPERIMENT1: Displacement, dissolving and neutralisation reactions
Example: magnesium displacing copper from copper(II) sulfate
- 50 cm3 of copper(II) sulfate is measured and transferred into a polystyrene cup.
- The initial temperature of the copper sulfate solution is measured and recorded.
- Magnesium is added and the maximum temperature is measured and recorded.
- The temperature rise is then calculated. For example:
Each type of chemical bond has a particular bond energy. The bond energy can vary slightly depending what compound the bond is in, therefore average bond energies are used to calculate the change in heat (enthalpy change, ΔH) of a reaction.
Example: dehydration of ethanol
Note: bond energy tables will always be given in the exam, e.g: