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, CuSO₄.?xH₂ODiagram: 

Heating a hydrated salt to remove the water of crystallisation

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 / M_r?? ? ? ? ? ? ? b / M_r

= y ???????????????????= x

Ratio ???????????????1 ?????????: ??????????x

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