Quantitative Analysis of a Soluble Sulfate

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Chem 102 - Lab #2
Quantitative Analysis
of a Soluble Sulfate

Steven English

Lab Instructor: Dr. Campo
Date: Tuesday, February 5th 2013

Pre-Lab Questions

A. Adding the acid to the sodium sulfate solution results in an increase in the solubility of any free anions present in the sample. This will happen because the present anions will bind with the hydrogen cations present in the acid.

B. The sodium sulfate is boiled because experiments have shown that barium sulfate is 50 times more soluble at 100°C than at room temperature. At the higher temperature, the Ba2+ and the SO42- are more likely to find each other and to form bonds and precipitate more slowly. If this happens too rapidly, then there is a high chance that impurities will be trapped in the precipitated crystals.

C. The sodium sulfate solution is kept hot for an hour in order to allow enough time for the entirety of the sulfate present in the solution to precipitate.

D. Digestion refers to the formation of the barium sulfate crystals that can then be filtered out of the solution

E. The crucible must remain uncovered both to ensure that there is good oxygen flow into the crucible ensuring that the burning off of the filter paper can be achieved, and also to allow the escape of the smoke and fumes that result from it. The oxygen flow also prevents the sulfur in the sample from being reduced.

Objective

This experiment has three main objectives:
Determining the amount of a substance, sulfate (SO42-), in a sample of sodium sulfate (Na2SO4) with soluble impurities Learn about the process of selective precipitation by seeing how barium sulfate (BaSO4) forms while other soluble materials remain in a solution To learn how to use gravimetric analysis to calculate the present sodium sulfate's mass by measuring the mass of the precipitated barium sulfate

Materials and Methods

Chemicals/Solutions:
Sodium sulfate (Na2SO4)
Distilled water (H2O)
0.25M Barium chloride solution (BaCl2)
Silver nitrate (AgNO3)

Equations for chemical reactions:
Na2SO4 (aq) + BaCl2 (aq) -----> BaSO4 (s) + NaCl (aq)

Equipment:
400 mL beaker
Analytical balance
Dropper
Bunsen burner
Buret
Watch glass
Cotton
Two crucibles with covers
Blue wax pencil
Ringstand with triangle and wire gauze
Funnel
Experimental Procedure:

A.
A 400 mL beaker was cleaned and dried
Approximately 0.5 grams of unknown sulfate solution was weighed and unknown selection's letter was written in the lab book Sample was added to beaker and weighed, and amount transferred into beaker was noted 50 mL of distilled water was added to beaker

1 mL concentrated HCl was added after sample dissolved
150 mL more distilled water was added and solution was heated to just boiling

B.
Buret with 50 mL of 0.25 M barium chloride solution was set up Heat was turned down to low as sodium sulfate began to boil 20 mL barium chloride solution was added drop by drop from buret to sodium sulfate solution with constant stirring and gentle heat Bunsen burner was turned off to allow precipitated barium sulfate to settle A few drops of barium chloride was added to test for complete precipitation 5 mL barium chloride drop by drop was added until precipitation was complete Beaker was covered with a watch glass once the precipitation was complete and insulated by wrapping with cotton Solution was set aside for one hour

C.
A filter paper cone was prepared as shown in lab manual and wetted it with distilled water Remaining supernatant liquid was decanted through filter paper Test outlined in step 5 of secton B was preformed to test for complete precipitation

D.
1. Precipitate remains in beaker were washed out with a small amount of distilled water 2. Crystals were added to those already in the funnel and washed 8-10 times with hot water, allowing excess water to drain 3....
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