Materials Included In Kit

Additional Materials Required

Safety Precautions

Barium nitrate solution is toxic by ingestion. Handle the crucible and its lid only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a hot crucible—remember that a hot crucible looks exactly like a cold one. Always keep your face at arm’s length from the crucible. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Have students wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Unused alum crystals may be stored indefinitely or disposed of according to Flinn Suggested Disposal Method #26a. The barium nitrate solution may be disposed of according to Flinn Suggested Disposal Method #27h. Provide containers labeled “Waste Container: Parts 1 and 2” in which students can place the used capillary tubes and the anhydrous alum. This waste may be disposed of according to Flinn Suggested Disposal Method #26a. Provide containers labeled “Waste Container: Part 3” in which students can place the precipitated barium sulfate and the filtrate. The filtrate will contain excess barium nitrate as well as potassium and aluminum ions. The filtrate may be disposed of according to Flinn Suggested Disposal Method #27h. The precipitated barium sulfate may be disposed of according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Review the recommended safety precautions and demonstrate the proper techniques for handling a crucible with crucible tongs and for heating a crucible in a Bunsen burner flame. The Flinn Scientific Laboratory Techniques Guide, AP6248, provides thumbnail illustrations of these and 14 other common laboratory techniques.
• Purchase closed end capillary tubes for the melting point determination (Flinn Catalog No. GP7047).
• The Thiele melting point tube is a convenient device to use for more accurate melting point determination, but a beaker can also be used. If a Thiele melting point tube is used, fill it with water to about 2 cm from the top. Position the thermometer bulb and crystals next to the side tube. Heat at the very bottom with a small flame from the Bunsen burner. Because the Thiele tube shape sets up convection currents, stirring is not needed. Do not stopper the end of the tube. Alternatively, a Mel-temp apparatus may be used.
• A rubber policeman attaches to a stirring rod and acts as a squeegee in helping remove the precipitate quantitatively from the beaker. Plastic policemen are also available. The derivation of the name is from the military use of the word “police” meaning to clean up an area.
• A watch glass and the bottom of a test tube, or mortar and pestle, can be used to pulverize the dry alum in Part 1.
• In Part 2, the water of hydration can also be driven off by heating the crucible containing the alum in the oven overnight at 110 °C.
• In Part 2, it is important to heat slowly at first so that the evolving water of hydration does not carry the alum crystals with it.
• Two of the most basic and important techniques to master in chemical analysis are filtering and decanting. These two techniques of quantitative transfer occur as crucial steps in many analytical determinations. To make sure the filtering speed is as rapid as possible, the filter paper must be seated properly in the funnel. The filter paper should make complete contact with the sides of the funnel and drain with few, if any, air bubbles in the stem. This is done by folding the filter paper as shown in Figure 9 and tearing off the corner. This allows the subsequent cone of filter paper to be placed smoothly against the sides of the funnel. Place the filter paper cone in the funnel. Wet the filter paper thoroughly with distilled or deionized water and use your fingers to smooth the paper against the sides of the funnel until no air bubbles are visible in the stem.
  {13801_Hints_Figure_9}
  Proper decanting technique ensures that the precipitate is transferred from the beaker to the filter paper and little, if any, is lost during the transfer. Start by holding a stir rod against the lip of the beaker and pour the liquid from the beaker into the funnel. The liquid should run down the rod and into the funnel without splashing (see Figure 10). Keep the level of the liquid in the funnel below the top of the filter paper.
  {13801_Hints_Figure_10}
  When all the liquid has been transferred to the funnel, begin transferring the remaining precipitate from the beaker to the funnel. Add a stream of distilled or deionized water to the beaker. Use a rubber policeman on the end of the stir rod to loosen any precipitate clinging to the beaker. Rinse the rubber policeman with a stream of distilled or deionized water, swirl the beaker to suspend the precipitate, and transfer the suspension to the funnel using the technique outlined in Figure 2. Repeat this rinsing until nearly all the precipitate has been transferred. Rinse the beaker again and transfer the liquid to the flask. Hold the beaker and stir rod as shown in Figure 11 and rinse the sides and bottom of the flask with distilled or deionized water. Rinse at a rate that allows the liquid to flow down the stir rod into the funnel without splashing and doesn’t allow the liquid level to rise above the top of the filter paper. Repeat this rinse until no precipitate is visible in the beaker.
  {13801_Hints_Figure_11}
  Instructors should demonstrate these techniques to the students and allow those not yet proficient sufficient time to practice setting up the filter paper in the filter cone and decanting a liquid from a flask.
• If no drying oven is available, an evaporating dish or large porcelain crucible and Bunsen burner can be used. Place the solid and paper in the evaporating dish or crucible and heat gently over a Bunsen burner for a few minutes. Make sure the paper is not exposed to the flame and do not allow the paper to char. In this manner, the moisture can be driven off without destroying either the filter paper or the precipitated barium sulfate.
• In Part 3, be sure to use quantitative filter paper (for example, Flinn Catalog No. AP8993, Quantitative Filter Paper) since the barium sulfate tends to form very fine crystals.

Answers to Prelab Questions

1. When measuring a melting point, why is it necessary to raise the temperature very slowly in the vicinity of the melting temperature? The temperature must be raised slowly in the vicinity of the melting point to give the thermometer time to come to equilibrium with the water and sample temperature.
2. Washing soda is a hydrated compound whose formula can be written Na₂CO₃•xH₂O, where x is the number of moles of H₂O per mole of Na₂CO₃. When a 2.123 g sample of washing soda was heated at 130 °C, all of the water of hydration was lost, leaving 0.787 g of anhydrous sodium carbonate. Calculate the value of x. Moles anhydrous sodium carbonate:
   {13801_PreLab Answers_Equation_1}
   Mass of water of hydration: 2.123 g – 0.787 g = 1.336 g H₂O
   Moles water:
   {13801_PreLab Answers_Equation_2}
   Ratio moles water : moles sodium carbonate:
   {13801_PreLab Answers_Equation_3}
   Correct formula: Na₂CO₃•10H₂O
3. The formula for Epsom salts is MgSO₄•7H₂O. If 1.250 g of the compound is dissolved in water, calculate the number of milliliters of 0.200 M Ba(NO₃)₂ that would be required to precipitate all of the sulfate ions as barium sulfate.
   {13801_PreLab Answers_Equation_4}
   Make the same determination for 1.000 g of alum.
   {13801_PreLab Answers_Equation_5}

Sample Data

Part 1. Melting Point Determination of Alum

Answers to Questions

Calculations and Analysis

Part 1. Melting Point Determination of Alum 

1. Find the literature value for the melting point of aluminum potassium sulfate and enter this value in the Part 1 Data Table.

   From the Merck Index, the melting point of alum is 92.5 °C.

1. From the mass of anhydrous alum remaining in the crucible after heating and its formula, AlK(SO₄)₂, calculate the moles of anhydrous alum in the original sample.
   {13801_Data_Equation_1}
2. From the mass of water driven off from the sample and the molar mass of water, calculate the moles of water evolved in the original sample.
   {13801_Data_Equation_2}
3. Calculate the mole ratio of water to anhydrous alum in the sample. Record this value in the Part 2 Data Table.

   5.20 x 10^–2 mol H₂O : 4.29 x 10^–3 moles AlK(SO₄)₂ = 12.1 : 1.0 ≈ 12 : 1

1. Calculated the percent sulfate in barium sulfate.
   {13801_Data_Equation_3}
2. Calculate the mass of sulfate in the precipitated barium sulfate. Record this value in the Part 3 Data Table.
   {13801_Data_Equation_4}
3. Calculate the percent sulfate in the alum sample. Record this value in the Part 3 Data Table.
   {13801_Data_Equation_5}
4. Calculate the theoretical percent sulfate in alum, AlK(SO₄)₂•12H₂O. Record this value in the Part 3 Data Table.

   Since there are two moles of sulfate in each mole of AlK(SO₄)₂•12H₂O:

   {13801_Data_Equation_6}
5. Calculate the percent error in the determination of sulfate ion in alum.
   {13801_Data_Equation_7}

Post-Laboratory Review

1. Why must objects be cooled before their mass is found on a sensitive balance?

   If an object is warm, it sets up hot air currents in the balance that rise and cause the object to weigh too little.

2. Comment on the results of the different tests used to verify that the substance tested was alum.

   In Part 1, the melting point was within 0.4 °C of the reported value indicating that the material probably was alum. In Part 2, the ratio of 12 moles of water to 1 mole anhydrous alum also corresponds with the correct formula. The percent sulfate found in Part 3 also correlates very closely with the theoretical percent.

3. What other tests could be made to verify the composition of alum?

   Methods to analyze the percent of aluminum or potassium in the crystal could also be used to verify the composition of alum.

Introduction

When a compound is synthesized, tests are carried out to confirm whether the compound formed is indeed the compound desired. There are a number of tests that can be performed to verify the identity of a compound. In this experiment several tests are carried out to determine if sample crystals are aluminum potassium sulfate (alum).

Concepts

• Percent composition
• Water of hydration
• Molecular formula

Background

Every compound has a unique set of chemical and physical properties. To identify a compound with certainty, a minimum number of these properties must be verified experimentally. In this experiment, three properties of a sample of aluminum potassium sulfate, AlK(SO₄)₂•12H₂O, are determined—its melting point, the number of moles of water of hydration in the formula, and percent composition of sulfate.

The first test in verifying the identity of alum is to find the melting point of the compound and compare it to the published value for alum. A small quantity of alum is powdered and placed in a capillary tube which is attached by a rubber band to a thermometer bulb. The crystals are heated in a water bath, and the temperature at which they melt is recorded and compared to reported values.

The alum is next analyzed for water of hydration. When an ionic compound is prepared in aqueous solution and isolated by crystallization, water molecules are often incorporated into the crystal structure of the compound in fixed proportions. The amount of water incorporated, referred to as the water of hydration, cannot be predicted for any compound, but must be determined experimentally. In order to determine the formula moles of water of hydration for alum, a portion of the alum will be placed in a crucible and weighed. The crucible is heated until all of the water of hydration is driven off. The crucible is then cooled and its mass measured. From the mass of the dry crystals and the mass of the water lost, the ratio of the moles of H₂O to the moles of AlK(SO₄)₂ is calculated and compared to the correct formula values.

The third test is a determination of the percent of sulfate in the compound. A weighed quantity of alum is dissolved in distilled water. An excess of barium ions is added to the solution to precipitate all of the sulfate as barium sulfate. The precipitated barium sulfate is filtered off, dried and its mass determined. From the mass of the barium sulfate and the initial mass of alum, the percent sulfate is calculated and compared with the theoretical percent found from the formula.

Experiment Overview

The purpose of the experiment is to analyze alum, AlK(SO₄)₂•12H₂O, by three techniques in order to verify its identity. The following properties will be determined—melting point, mole ratio of hydrated water to anhydrous potassium aluminum sulfate, and percent of sulfate ion contained in the compound. Each of these properties will be compared to the literature or calculated values for alum.

Materials

Prelab Questions

1. When measuring a melting point, why is it necessary to raise the temperature very slowly when approaching the melting temperature?
2. Washing soda is a hydrated compound whose formula can be written Na₂CO₃•xH2O, where x is the number of moles of H₂O per mole of Na₂CO₃. When a 2.123 g sample of washing soda was heated at 130 °C, all of the water of hydration was lost, leaving 0.787 g of anhydrous sodium carbonate. Calculate the value of x.
3. The formula for epsom salts is MgSO₄•7H₂O. If 1.250 g of the compound is dissolved in water, calculate the number of milliliters of 0.200 M Ba(NO₃)₂ that would be required to precipitate all of the sulfate ions as barium sulfate.

   
   Make the same determination for 1.000 g of alum.

Safety Precautions

Barium nitrate solution is toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Thoroughly wash hands with soap and water before leaving the laboratory.

Procedure

Part 1. Melting Point Determination of Alum

1. Using a mortar and pestle, pulverize a small amount (about 0.5 g) of dry alum.
2. Pack the alum in a capillary tube to a depth of about 0.5 cm. To get the alum into the capillary tube, push the open end of the capillary tube down into a small pile of alum powder.
3. To pack the alum tightly at the closed end of the capillary tube, turn the tube so the open end is up, and bounce the bottom of the tube on the desk top.
4. Fasten the capillary tube to a thermometer with a rubber band. The alum should be level with the thermometer bulb (see Figure 1).
   {13801_Procedure_Figure_1}
5. Using a universal clamp and cork stopper (or split rubber stopper), fasten the thermometer to a ring stand.
6. Immerse the bottom of the capillary and thermometer in a beaker of water (or a Thiele melting point tube filled with water) and heat (see Figure 2). If using a beaker, stir the water to maintain an even distribution of temperature. The water bath may be heated rapidly in the beginning, but as the temperature approaches the melting point of alum, the water bath should be heated more slowly in order to get an accurate temperature reading of the melting point. Alternatively, a Mel-temp apparatus may be used.
   {13801_Procedure_Figure_2}
7. Record the temperature range at which the alum melts (the white powder will turn to a colorless liquid) in the Part 1 Data Table.
8. Repeat the melting point determination, using a fresh sample of alum and a new capillary tube. 
9. Place the used capillary tubes and the anhydrous alum in the waste container marked for Part 1.

Part 2. Determination of the Water of Hydration in Alum Crystals

1. Set up a Bunsen burner on a ring stand beneath a ring clamp holding a clay pipestem triangle (see Figure 3). Do NOT light the Bunsen burner.
   {13801_Procedure_Figure_3}
2. Adjust the height of the ring clamp so that the bottom of a crucible sitting in the clay triangle is about 1 cm above the burner. This will ensure that the crucible will be in the hottest part of the flame when the Bunsen burner is lit.
3. Place a crucible with a cover in the clay triangle and heat over a burner flame until the crucible is red hot.
4. Turn off the gas source and remove the burner.
5. Using tongs, remove the crucible cover and place it on a wire gauze on the bench top. With the tongs, remove the crucible from the clay triangle and place it on the wire gauze as well (see Figure 4).
   {13801_Procedure_Figure_4}
6. Allow the crucible and its cover to cool completely on the wire gauze for at least 10 minutes.
7. Find their mass using an analytical balance. Handle with tongs or forceps to avoid getting fingerprints on the crucible and lid.
8. Record their mass in the Part 2 Data Table.
9. Now add about 2 g of alum crystals to the crucible. Weigh the crucible, cover and crystals and record their combined mass in the Part 2 Data Table.
10. Set the crucible at an angle in a triangle held in a ring on a ring stand. Cover the crucible loosely with the crucible cover, and heat very gently. The alum crystals will melt, and the water of hydration will evaporate. It is important that the escaping vapor does not carry any of the anhydrous alum along with it, so be sure that the crystals are heated very gently at first (see Figure 5).
    {13801_Procedure_Figure_5}
11. After the bubbling has stopped, heat the sample more strongly for an additional five minutes.
12. Turn off the gas source and remove the burner.
13. Using tongs, remove the crucible cover and place it on a wire gauze on the bench top. With the tongs, remove the crucible from the clay triangle and place it on the wire gauze as well (see Figure 4).
14. Allow the crucible and its cover to cool completely on the wire gauze for at least 10 minutes.
15. Measure and record the mass of the crucible, cover and anhydrous alum.
16. Repeat the drying procedure until constant mass is obtained.
17. Record the final mass of the crucible, cover and anhydrous alum in the Part 2 Data Table.
18. Dispose of the anhydrous alum according to your instructor’s directions. Carefully clean the crucible and crucible cover.
19. Place the used capillary tubes and the anhydrous alum in the waste container marked for Part 2.

Part 3. Determination of the Percent Sulfate in Alum

1. Use an analytical balance to accurately weigh about 1 gram of alum into a clean 250-mL beaker.
2. Dissolve the alum in approximately 50 mL of distilled water.
3. Place the beaker on the hot plate and heat the solution to 90 °C.
4. Obtain 50 mL of the 0.200 M Ba(NO₃)₂ solution in a clean 50-mL graduated cylinder.
5. Add twice the volume of the 0.2 M Ba(NO₃)₂, calculated in the Prelab Question 3, slowly into the alum solution, while stirring.
6. Cover the beaker with a watch glass and heat the solution nearly to boiling. Keep the solution just under the boiling point for at least 15 minutes. During this time, the precipitate particles should grow to a filterable size. (Alternatively, cover the beaker and allow the precipitate to stand overnight.)
7. Obtain a piece of quantitative filter paper. Weigh the filter paper on the analytical balance. Record the mass of the filter paper in the data table.
8. Fold the filter paper into a cone. First fold the filter paper in half and crease. Next, fold the filter paper almost in half again, leaving about a 5° angle between the folded edges (see Figure 6).
   {13801_Procedure_Figure_6}
9. Tear off the corner of the top edge, open the filter paper into a cone shape, and place the torn corner in the bottom of the cone.
10. Place the cone into the filter funnel. Position the paper tight against the funnel walls and moisten the paper with about 5 mL of deionized water from a wash bottle. Note: After adding the water, use index fingers to seat the filter paper tightly against the sides of the funnel so that little, if any, air gaps are visible in the stem as the water filters through.
11. Set up the ring stand and iron ring and place the funnel in the ring. Let the funnel drain into a second 400-mL beaker (see Figure 7).
    {13801_Procedure_Figure_7}
12. Using a stirring rod, decant the liquid from the 400-mL beaker into the funnel. Be sure to keep the liquid level below the top of the filter paper cone (see Figure 8).
    {13801_Procedure_Figure_8}
13. When all but approximately 10 mL of the liquid has been transferred, swirl the beaker to suspend the precipitated BaSO₄. Transfer this to the funnel, again making sure not to fill the cone above the top of the filter paper.
14. Rinse the flask with small amounts of distilled or deionized water from the wash bottle and then transfer the washings to the filter.
15. When all the solid has been transferred to the filter paper, rinse the solid with three small portions of distilled or deionized water. Allow the funnel to drain completely.
16. Obtain a watch glass. Using a microspatula, take the filter paper out of the funnel and place it in the center of the watch glass. Be careful not to tear the paper or to lose any part of the solid.
17. Using the microspatula, carefully open the filter paper into a circle on the watch glass. Place the watch glass and filter paper in a drying oven set at 110–120 °C.
18. Allow the filter paper to dry for 10–15 minutes. Remove the watch glass from the oven using crucible tongs. Use the spatula to break up the BaSO₄ into small particles.
19. Return the watch glass to the drying oven for an additional 5 minutes.
20. Remove the watch glass from the oven and set it aside to cool.
21. When cool, weigh the filter paper and the solid BaSO₄ on an analytical balance. Record the mass in the data table.
22. Repeat steps 19–21, until the mass readings do not change by more than 0.005 g.
23. Place the precipitated barium sulfate and the filtrate in the waste containers marked for Part 3. The filtrate will contain excess barium nitrate as well as potassium and aluminum ions.

Student Worksheet PDF

13801_Student1.pdf