# EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：1.5.6 Experiment: Finding Formulae of Compounds

### Formulae of Simple Compounds by Experiment

#### Making Careful Quantitative Measurements

• The formulae of simple compounds can be found by careful experimentation and accurate measurements of mass changes
• The principle is to use mass measurements before and after reaction and then convert masses into moles
• Using the moles of reactants and products it is possible to deduce molar ratios and hence an empirical formula
• Experiments which are easier to do using this process involve gases being lost or gained
• In this example a hydrated salt is heated to drive off the water as water vapour

#### The Formula of a Hydrated Salt

Aim:

To determine the formula of hydrated copper sulfate, CuSO4xH2ODiagram:

Heating a hydrated salt to remove the water of crystallisation

Method:

• Measure mass of evaporating dish
• Add a known mass of hydrated salt
• Heat over a Bunsen burner, gently stirring, until the blue salt turns completely white, indicating that all the water has been lost
• Record the mass of the evaporating dish and contents

Practical tip:

Avoid overheating the salt as it could decompose and give you a larger mass change

Mass of the white anhydrous salt:

Measure mass of white anhydrous salt remainingMass of water:Subtract mass of the white anhydrous salt remaining from the mass of known hydrated salt

Step 1 – Divide the mass of the copper sulfate and the water by their respective molar masses

Step 2 – Simplify the ratio of water to copper sulfate

anhydrous salt      water

Mass               a                       b

Moles              a / Mr               b / Mr

= y                    = x

Ratio                1          :           x

Step 3 – Represent the ratio in the form ‘salt.xH2O’

#### Exam Tip

It is unlikely that you will get a whole number for the number of moles of water in the ratio, so you will need to round up or down to the nearest whole number.