Gravimetric and Volumetric Analysis

Which equation do you use to calculate number of mole given mass?

What is the difference between molecular and empirical formulae?

What is the conventional unit for mass in chemistry?

Getting Ready...

It will be very helpful to review: Common chemical reactions eg. acid + metal

carbonate, combustion. Solubility rules and precipitation reactions.

Getting Ready...

o Filtrateo Residueo Precipitateo Mass

Key Terms

o Empirical formulao Molecular formulao % by masso Excess reactants

What the heck is he

talking about?

Analysis using the mass of a substance. Simplest version is analysing for water

content.

Gravimetric Analysis

A 6g sample of hydrated copper sulphate is heated overnight in an incubator at 110°C. The next day the sample weighs 3.84 grams. What was the ratio of water to copper sulphate in the original sample?

Gravimetric Analysis

More involved analyses involve a reaction to create a precipitate followed by filtration.

Gravimetric Analysis

Arrange the following steps of gravimetric analysis into the correct order.

Filter out precipitate. Add reactant. Dry and weigh residue in filter paper. Dissolve sample in water. Weigh the sample.

Gravimetric Analysis

Gravimetric Analysis

Gravimetric analysis can be used to determine the sulphate content of fertilisers. A 2.50 g sample of a fertiliser was dissolved in water. Barium sulphate was precipitated by adding excess barium chloride solution. The mass of barium sulphate obtained after drying was 0.50g.

Write the equation for this reaction. Calculate the mass of sulphate in the sample. What is the percentage by mass of sulphate in the

fertiliser?

Example Problem

Write the equation for this reaction. Calculate the mass of sulphate in the sample. What is the percentage by mass of sulphate in the

fertiliser?

Example Problem

Mass of fertiliser = 2.50gMass of barium sulphate = 0.50g

If you know the percentage mass of the constituent elements of a compound (eg. 88.9% oxygen, 11.1% hydrogen), how do you work out the empirical formula?

What other piece of information do we then need to determine the molecular formula?

Empirical and Molecular Formulae

A 5.16g sample of a hydrocarbon was analysed and determined to contain 2.69g of carbon, 0.67g of hydrogen, and some oxygen. In a separate test the molar mass of the compound was found to be 92.0g/mol. Determine the molecular formula of the compound.

Empirical and Molecular Formulae

5.16g sample 2.69g of carbon, 0.67g of hydrogen 92g/mol

Empirical and Molecular Formulae

Your precipitate is slightly soluble.

There is another species present which also forms a precipitate.

You add too much of the precipitate causing reactant.

When things go wrong…

Image Credits

‘Copper sulphate’ By Stephanb (Own work) (http://commons.wikimedia.org/wiki/File:Copper_sulfate.jpg), via Wikimedia Commons