1.15 Qualitative Chemical Analysis pp

Qualitative Chemical Analysis Allows us to determine whether a chemical is present in the sample It is qualitative because it does not tell us information about how much (quantity) is present We can use solubility rules to determine the presence of certain ions in a solution

Using Solubility Rules If you think sulfate ions (SO 4 2- ) are dissolved in a sample of water, what ions can we add to the solution to test for sulfate? – Ca 2+, Sr 2+, Ba 2+, Pb 2+, Ra 2+ If we add one of these ions to the sample and it precipitates, we know that sulfate ions are present – This is a positive test

Testing for Multiple Ions Most solutions contain more than one type of ion To do more than one test on the same sample takes careful planning: 1.Order is important – If I suspect Ca 2+ and/or Ba 2+, you will not know which is present if you add SO 4 2- – You will need to react the solution with something that will react with only one and ensure all of that ion precipitates out of the solution (e.g. OH - )

Testing for Multiple Ions 2.If a precipitate forms, it must be removed from the sample before doing a second test – This can be done with a centrifuge – a device that spins the sample at high speed to separate particles by density – The precipitate will sink to the bottom and the remaining solution (supernate) can be removed and tested

Solution containing Fe 2+ (aq) and/or Sr 2+ (aq) ions precipitate add NaOH(aq) no precipitate Solution contains Fe 2+ ions, precipitated as Fe(OH) 2(s) centrifuge No Fe 2+ ions present Fe(OH) 2(s) supernate Add Na 2 SO 4(aq) precipitate Solution contains Sr 2+ ions, precipitated as SrSO 4(s) no precipitate No Sr 2+ ions present