Gravimetric Analysis of a Sulfate Mixture

Objective

To determine the relative percentages of sodium and potassium sulfates in an unknown mixture

Concepts

Gravimetric analysis, stoichiometry, mole ratios

Introduction

In this experiment you will be given a sample that contains sulfate ion. It is a mixture of two compounds, sodium sulfate and potassium sulfate, both anhydrous salts. Your first task is to determine the total number of moles of sulfate ion in the original mixture. From that quantity and using the total mass of your sample and the molar masses of the two components of the mixture, you will be able to determine the composition of the sulfate mixture.

The precipitate is collected by centrifugation, washed carefully, dried, and weighed. This experiment is an example of gravimetric analysis. You can expect to spend about an hour on the first day, plus short periods of time on one or more successive days, perhaps while another experiment is in progress.

Prelaboratory Assignment

1. Read the entire experiment before coming to the laboratory.

2. Prepare a data table for recording masses and descriptive observations.

Prelaboratory Questions (click here for help)

1. Write the balanced molecular and complete-ionic equation for the reaction between barium chloride and potassium sulfate in aqueous solution to form barium sulfate. Show that the net-ionic equation for this reaction is the same as was given above.

2. Suggest a reason why it is desirable to use an excess of barium ion in the precipitation.

3. A sample of an unknown sulfate compound has a mass of 0.1000g. Addition of excess barium chloride solution to the sample forms a barium sulfate precipitate of mass 0.0676g. What is the mass percent of sulfate ion in the unknown compound?

4. Hydrated calcium sulfate, CaS04, contains of 55.8 sulfate ion, by mass. Calculate the number of waters of hydration that are present in the hydrate. Write the correct formula for the hydrate.

Safety Precautions

1. Chemical splash-protective eyewear must be worn at all times in the laboratory.

2. Hydrochloric acid is corrosive to skin and clothing.

3. Barium compounds are toxic. Wash thoroughly with soap and water before leaving the laboratory.

Materials

Procedure

For the most precise results, duplicate determinations should be carried out, each following the sequence of steps given below. The sample mass should be in the range of 100-150 mg if a balance with milligram sensitivity is used, or 50-100 mg if an analytical balance is available.

Note: If you were given a numbered unknown sample, be sure to record the number in your notebook.

1. Determine the mass of a clean, dry 100-mm test tube. Place a small amount of the unknown
(about the volume equivalent of one grain of rice) in the tube, and then reweigh the tube and contents. Add about 1 mL of distilled water to dissolve the sample, gently warming the tube on a hot plate or carefully in a burner flame if necessary. If the mixture is heated too strongly, it may "bump" and some of the liquid may be lost. Some solids may require more water, but be careful not to fill the tube beyond about 1/3 full. The sample must be completely dissolved before proceeding. Add 2 drops of 1 M HCl to the tube and shake gently. Follow this with 0.5-1.0 mL of 1.0 M BaCl2 added dropwise, with gentle shaking after each 2-3 drops. Take care not to get barium chloride on your skin; if you do, wash it off with soap and water. Warm the tube and contents on a hotplate for 2-3 minutes to aid coagulation of the precipitate. Do not boil.

2. Remove the tube from the heat, allow it to cool, then centrifuge for 30 seconds. Without disturbing the solid on the bottom of the test tube, add one more drop of the barium chloride solution. If no new cloudiness appears, proceed with the washing of the precipitate (step 4). If cloudiness is observed, add five more drops of the BaCl₂, then heat, centrifuge, and test again with barium chloride; continue in this fashion until addition of barium chloride does not cause further cloudiness.

3. Decant and discard the clear supernatant solution above the barium sulfate precipitate, being careful not to lose any solid. A microtip transfer pipet or Pasteur pipet is useful for this purpose. Add about 10 drops of ice-cold distilled water to the solid in the tube, then shake the tube and contents until all of the precipitate is suspended in the water (it will not dissolve). Centrifuge the suspension and again discard the clear, colorless supernatant, being careful not to lose any of the white solid. Repeat the ice-water rinse, centrifuging, and decanting twice more, followed by a final rinsing with acetone.

4. Dry the test tube and precipitate first on a steam bath to drive off most of the acetone and then in a 100-110°C oven for at least one hour, preferably overnight.

5. Remove the tube from the oven and allow it to cool for a minimum of 30 minutes in a desiccator. Determine the mass of the tube and contents. Return the tube to the oven for at least an hour, cool in the desiccator once more, then reweigh it. If the mass agrees within experimental error (± 0.5 of the precipitate mass) with the previous value, the experiment is completed. If not, continue the cycle of oven-drying, cooling, and weighing until a constant mass is obtained.

Disposal

1. Barium sulfate, like barium chloride, is toxic by ingestion. Transfer your solid product to an appropriate container for removal as hazardous waste. Small amounts of barium ion were washed into the effluent stream during the decanting and rinsing portions of the procedure; this is unavoidable, but the amounts are below the parts-per-trillion level, so present no significant hazard to the environment.

2. Wash all glassware immediately after use and return it to its proper location.

Processing the Data (click here for help)

1. Determine each of the following, showing appropriate calculations to support your results. If you did two or more trials, as was suggested, you need only show calculations for one of the trials, with results for all presented in the form of a table, with separate columns for each sample.

· Mass of original sulfate sample

· Mass and moles of barium sulfate produced

· Moles and mass of sulfate ion present in unknown sample

· Mass percent of sulfate ion in unknown

2. The unknown that you used was a mixture of potassium sulfate, K₂S0₄, and sodium sulfate, Na₂S0₄. Here's what you know:

· The total number of moles of barium sulfate equals the number of moles of sulfate ion in the original sample, which, in turn, must equal the combined moles of K₂S0₄ and Na₂S0₄.

· The total mass of the original sample must be the combined masses of K₂S0₄ and Na₂S0₄.

· The mass of K₂S0₄ is found by multiplying the number of moles of K₂S0₄ by the molar mass of K₂S0₄; likewise, the mass of Na₂S0₄ is found by multiplying the number of moles of Na₂S0₄ by the molar mass of Na₂S0₄.

· The number moles of Na₂S0₄ must be the difference between the total moles of sulfate and the number of moles of K₂S0₄ in the original sample.

Given this information it is a simple matter of algebra to determine the masses of K₂S0₄ and Na₂S0₄ that were in the original sample, and from those you can establish the mass percent composition of the sample mixture. Thus, if the number of moles of barium sulfate recovered is, say, 7.50 x 10-⁴ mol, and if the number of moles of K₂S0₄ is represented by x, then the number of moles of Na₂S0₄ would be (7.50 x 10-⁴ -x). If you carried out multiple trials, you need only show the actual calculations for one trial; simply report the masses ofK₂S0₄ and Na₂S0₄, and the percentage composition of the mixture for each additional trial.

Analysis and Conclusions (click here for help)

1. Suggest explanations for each of the following parts of the procedure.

a. The first three rinsings of the product specified the use of distilled water that was ice-cold, rather than room temperature.

b. Acetone is used for the final rinse.

2. What would be the effect on your determination of the mass of barium sulfate in each of the following cases? For each one, you are to decide whether the reported mass ofBaS04 would be too high, too low, or unaffected. Explain the reasoning behind your choice.

a. Too little barium chloride solution was used.

b. The sample was not thoroughly dried.

c. The tube and contents were not cool before the final weighing.

3. The purpose of adding hydrochloric acid in Part B is to remove any carbonate ions that might be present in the unknown, so that barium carbonate will not precipitate along with the sulfate. Write the net-ionic equation for the reaction between protons (hydrogen ions) in solution and dissolved carbonate ions. What effect on your sulfate percentage could result if the addition of acid were omitted? Explain.

4. What effect on your sulfate percentage could result if the addition of acid were omitted? Explain.

5. Error analysis: Discuss experimental errors and identify those portions of the procedure where extra care is needed to ensure satisfactory results.