A.P. Chemistry                                                 Name_____________________________

Gravimetric Analysis Lab

Mr. McAfoos                                                               Period________ Date________________

 

In this lab you will be determining the percent sodium sulfate and the percent potassium sulfate in a mixture of the two using gravimetric analysis.

 

Normally, to determine the amount of an ion present in a solution we can use a form of titration, where we systematically add another solution until a reaction occurs that depends on the presence or absence of particular ion.  The most common example would be acid/base titrations that involve a color change when the amount of hydronium (H+) ions becomes either higher or lower than a certain threshold value.

 

When determining the amount of certain ions however this method does not work.  Sulfates, for example, tend to be highly insoluble with a wide variety of cations (positive ions).  This dramatically limits our ability to have other ions present in solution that will react after the sulfate is used up.  We can, however, use the high insolubility of sulfate salts to our advantage.

 

We will be determining the amount of sulfate ion present in a mixture of potassium and sodium sulfate.  From this data you will be able to determine the percent of each sulfate salt in the original mix.  The sample will be dissolved and the sulfate precipitated out with barium ions.  The solid precipitate will be digested and then filtered through quantitative filter paper.  The filter paper will then be dried in an oven and reweighed. By determining the mass of the solid, you will be able to determine the percent sulfate in the initial sample and therefore the percent of the constituent salts.

 

Procedure: 

PREPARATION OF SAMPLE:  Take approximately 2 g of the unknown sample and weigh it to the nearest 0.001 g.  Dissolve it in about 50 ml of deionized water in a 400mL beaker. 

 

Measure 100 ml of saturated barium nitrate solution. If the solution is cloudy, you will need to filter it before using. Add this to the first solution.  There should be an immediate precipitation of barium sulfate.

 

DIGESTION:  Heat the solution and solid over a low flame for at least thirty minutes.  This will cause some of the solid to dissolve and re-precipitate.  Since the smallest particles are the ones most likely to dissolve, and since re-precipitation tends to happen in contact with already present precipitate, you will be increasing the average size of the particles in the beaker.  WARNING:  boiling solutions with solids tend to bump, that is to bubble suddenly and violently.  Stir regularly and avoid overheating the solution.

 

FILTRATION:  Filter the solution through a piece of pre-weighed quantitative filter paper and rinse any solid that remains in the beaker into the filter paper.  The filtrate should be clear.  If it is not, you should re-digest the filtrate and re-filter through a new piece of filter paper. Add a small amount (a few drops) of filtered barium nitrate solution to the filtrate.  If precipitation occurs, add 20 ml of filtered barium nitrate solution, redigest and filter this through a new piece of filter paper.  Retest the filtrate.

 

FINISHING:  Carefully remove the wet filter paper and barium sulfate from the funnel and lay the filter paper on a clean watch glass. Place the watch glass in the oven on a piece of paper with your names on it. Leave the filter paper and barium sulfate in the oven until dry, then reweigh. Reheat the paper and reweigh it to ensure complete dryness.

 

Data analysis:

Determine the mass of the barium sulfate recovered, and the mass of sulfate present in your original sample.  Set up an algebraic formula to determine the percent of the sulfate salts in the original mixture.