Gravimetric and Volumetric Analysis

Getting Ready... 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.

What the heck is he talking about? Key Terms Filtrate Residue Precipitate Mass Empirical formula Molecular formula % by mass Excess reactants What the heck is he talking about?

Gravimetric Analysis 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

Example Problem 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? Mass of fertiliser = 2.50g Mass of barium sulphate = 0.50g

Empirical and Molecular Formulae 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

When things go wrong… Your precipitate is slightly soluble. There is another species present which also forms a precipitate. You add too much of the precipitate causing reactant.

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