Q. Read Exp 2 and use your own words to summarize the lab 2 ?

Exp 2- Quantitative Analysis of a Soluble Sulfate

Introduction:

The amount of Na2SO4 in a sample of unknown composition will be determined gravimetrically in this experiment. Sodium sulfate is soluble in aqueous solution as are most of the impurities found in the Na2SO4 sample. Acidification with HCl will increase the solubility of any other possible anions present in the sample. The SO42- will be precipitated as BaSO4 as a solution of BaCl2 is added; BaSO4 is one of the few barium salts that is insoluble under acidic conditions. The following reaction occurs:

Na2SO4(aq) + BaCl2(aq) g BaSO4(s) + 2 NaCl(aq)

BaSO4 is 50 times more soluble at 100C in aqueous solution than at room temperature. Addition of BaCl2 solution to the boiled Na2SO4 solution allows the Ba²⁺ and SO42- "to meet" and precipitate slowly; this process is called digestion.

Slow precipitation allows for the formation of relatively large pure crystals. As crystals form slowly the soluble ions are excluded from the precipitate; conversely, when crystals form rapidly, impurities are trapped within the crystal matrix. A well digested precipitate should settle rapidly when stirred leaving a clear liquid above the crystals. Phosphates, carbonates and other impurities should remain in the liquid.

Procedure:

WEAR YOUR SAFETY GLASSES WHILE PERFORMING THIS EXPERIMENT

Clean and dry a 400 mL beaker. Obtain an unknown sulfate sample and record the letter on the sample vial label in your lab notebook and/or data sheet. Accurately weigh out approximately 0.5 grams of your unknown sulfate sample on weighing paper or in a weighing boat; if weighing boat or weighing paper is zeroed, ignore #1 and #2 in your data sheet. Add 50 mL of distilled water to the beaker. If the sample dissolves completely, add 1 mL of concentrated HCl. If the sample does not dissolve, add HCl dropwise with stirring until the sample dissolves (impurities are being dissolved at this point); then add 1 mL HCl in excess. Add about 150 mL more distilled water and heat the solution using a hot plate, until it is just boiling.

As the solution is heating, set up a buret with 25 mL of 0.25M BaCl2 solution. When the Na2SO4 has just boiled, turn the hot plate off and add the aqueous BaCl2 drop-by-drop with constant stirring until 20 mL have been added. Let the precipitated BaSO4 settle gradually. Test the supernatant liquid (liquid above the crystals) for completeness of precipitation by adding a few drops of BaCl2.

If the solution turns cloudy or turbid after the addition, add 5 mL more of the BaCl2 solution dropwise with stirring. Let the precipitate settle and test it for completeness again. When the precipitation is complete, cover the beaker with a watch glass and keep it hot. Allow the solution to sit until the beaker is just warm to the touch.

Weigh a piece of filter paper provided by your instructor and record the mass on the data sheet. Prepare a filter paper cone by folding the filter paper in half, then fold in half again, and pick a side to open. Place the filter paper in a glass funnel and then place that in a 250 ml Erlenmeyer flask. Wet the filter paper with deionized water so that it adheres to the sides of the glass funnel.

After the sample has digested the supernatant above the precipitate should be perfectly clear. Hot liquids filter faster than cold liquids so it is advantageous to filter immediately following digestion. Decant the clear liquid through the filter paper; be careful not to let the solution fill more than half of the filter paper cone. The precipitate will tend to "creep" to the top of the solution and could wash over the filter paper if the funnel is filled too full.

Use a small amount of water to rinse and to clean the last crystals out of the beaker; add this to the precipitate in the funnel. Wash the precipitate on the filter paper several times with water from a wash bottle. Allow the water to drain between washings.

Carefully remove the filter paper from the funnel, place in a small (100 mL) beaker and give to your lab instructor, label the small beaker with your name. The sample will be weighed the following lab meeting and percent sulfate determined as in the example calculation below.

Sample Calculation

%SO42-=Mass BaSO4 g*MW SO42-(gmol) MW BaSO4 (gmol)Mass Na2SO4 (g)*100%