Materials Included In Kit

Additional Materials Required

Prelab Preparation

Unkown Solid Samples

1. Mass a clean, dry weighing bottle on an analytical balance. Record the mass. Make sure the bottle can contain all the barium nitrate and sodium nitrate.
2. Add all the sodium nitrate to the weighing bottle and mass on the analytical balance. Record the mass.
3. Add all the barium nitrate to the bottle and mass on the balance. Record the mass.
4. Thoroughly mix the sample.
5. Calculate the percent barium in the sample.
   {12084_Preparation_Equation_3}
6. Dispense the mixture in approximately 1 g amounts in each of the 12 test tubes. Cork the samples and label test tube with marker. Save the remaining solid mixture.

Safety Precautions

Barium nitrate and sodium nitrate are strong oxidizers and moderately toxic by ingestion. Sulfuric acid is toxic by ingestion and is corrosive to eyes, skin and body tissue. Avoid all body contact. Wear chemical splash goggles and chemical-resistant gloves and 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. The solid sample mixture and the barium sulfate precipitate may be disposed of according to Flinn Suggested Disposal Method #27h. The filtered solutions may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this Super Value Kit for 5 classes of 24 students each, working in pairs (60 total student groups).
• The actual laboratory work for this experiment takes only about 50 minutes, but most students will require additional time. If more than one lab period is needed, a convenient stopping point is Part II, step 11. The watch glasses and precipitates may be left in the drying oven overnight.
• 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 4 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.
  {12084_Hints_Figure_4}
• 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 5). Keep the level of the liquid in the funnel below the top of the filter paper.
  {12084_Hints_Figure_5}
• 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 5. 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 6 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.
  {12084_Hints_Figure_6}
  Teachers should demonstrate these techniques to the students and allow sufficient time for students 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.
• Ideally, the evaporating dish should be slightly larger than the filter paper.
• Qualitative filter paper must be used or else the barium sulfate will not be completely captured by filtration.

Teacher Tips

• The solution is first warmed to allow the BaSO₄ precipitate to form more slowly and more perfectly than in cold water. If the precipitate forms too quickly, the barium sulfate is too small to be completely captured by the filter paper. This step is critical for good yield.
• The process of letting the precipitate develop in a hot solution is called digestion. This allows the crystals to grow in size, making collection by filtration easier and reducing losses.

Answers to Prelab Questions

An unknown sample was analyzed gravimetrically and yielded the following data.

1. From the mass of BaSO₄, calculate the number of moles of BaSO₄.
   {12084_Answers_Equation_4}
2. The moles of BaSO₄ precipitated are equal to the moles of barium in the sample. Calculate the mass of barium in the sample.
   {12084_Answers_Equation_5}
3. Calculate the percent barium in the sample.
   {12084_Answers_Equation_6}

Sample Data

Answers to Questions

1. Calculate the moles of precipitated barium sulfate, BaSO₄. Enter this value in the data table.
   {12084_Answers_Equation_7}
2. The moles of precipitated barium sulfate are equal to the moles of barium in the original sample. Calculate the mass of barium in the original sample. Enter this value in the data table.
   {12084_Answers_Equation_8}
3. From the calculated mass of barium and the mass of the sample, calculate the percent barium in the sample. Enter this value in the data and results table.
   {12084_Answers_Equation_9}
4. Review the procedure and list possible sources of errors that would cause the percent of barium in the unknown to be (a) too high or (b) too low.
   1. Any steps that would result in the mass of the unknown being too high (i.e., the unknown and weighing boat were not properly massed).

      Any steps that would result in the moles of barium sulfate being too low (i.e., barium sulfate solid lost in any of the steps between filtering the precipitate and weighing the dried filter paper and solid).

   2. Any steps that would result in the mass of the unknown being too low (i.e., if the unknown and weighing boat were not properly massed).

      Any steps that would result in the moles of barium sulfate being too high (i.e., the filter paper and barium sulfate were not completely dry) in Part II, step 16 when measuring the final mass.

Introduction

How do chemists determine the identity of a compound? A large variety of analytical techniques and procedures, ranging from instrumental methods such as spectroscopy and chromatography to more classical methods, such as titration and gravimetric analysis, have been developed to accomplish this task. In this activity, the percent barium in a solid sample is determined gravimetrically using a double replacement precipitation reaction.

Concepts

• Double replacement reaction
• Gravimetric analysis

Background

In this experiment, an unknown salt sample is analyzed to determine the percent composition of barium. A known amount of the sample is dissolved in water to dissociate the compound into its ions in solution. The barium in the sample is in the form of barium nitrate. It dissociates in water to form:

Sulfuric acid, H₂SO₄, is then added to this solution. The barium is precipitated out of solution as barium sulfate. The overall reaction represents a double replacement reaction with a precipitate formed.

Ba(NO₃)₂(aq) + H₂SO₄(aq) → 2HNO₃(aq) + BaSO₄(s)

The precipitated barium sulfate is filtered, dried and weighed. The moles of barium sulfate precipitated is equal to the mass of barium sulfate divided by its molar mass. This mole value represents the number of moles of barium contained in the original sample. Multiplying this mole value by the atomic mass of barium gives the grams of barium in the sample. Dividing the mass of barium by the mass of the original sample gives the percent barium present in the unknown sample.

Experiment Overview

The purpose of this lab is to determine the percent by mass of barium in an unknown solid mixture by gravimetric analysis. The unknown is weighed and dissolved in water. Sulfuric acid is added to the solution to precipitate the barium in the solution as barium sulfate. The precipitate is filtered, dried and weighed. From the data, the percent barium in the unknown is determined.

Materials

Prelab Questions

An unknown sample was analyzed gravimetrically and yielded the following data:

1. From the mass of BaSO₄, calculate the number of moles of BaSO₄.
2. The moles of BaSO₄ precipitated are equal to the moles of barium in the sample. Calculate the mass of barium in the sample.
3. Calculate the percent barium in the sample.

Safety Precautions

The unknown solid is moderately toxic by ingestion and is a strong oxidizer. Sulfuric acid is toxic by ingestion and is corrosive to eyes, skin and body tissue. Avoid all body contact. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part I. Precipitating Barium Sulfate

1. Obtain a sample test tube from the instructor.
2. On an analytical balance, mass a weighing boat. Record the mass in the data table.
3. Transfer the sample from the vial to the weighing boat. Mass the weighing boat and the sample on the balance. Record the combined mass of the weighing boat and sample in the data table.
4. Add the sample to a clean 400-mL beaker.
5. Add about 200 mL of distilled or deionized water to the beaker and stir to dissolve the sample.
6. Place the beaker on a hot plate and heat the solution almost to boiling.
7. Remove the beaker from the hot plate using heat-resistant gloves and place it on the heat-resistant pad.
8. Obtain 30 mL of 3 M H₂SO₄ solution in a 50-mL graduated cylinder.
9. Slowly add the 30 mL of 3 M H₂SO₄ to the warm solution while stirring constantly.
10. Allow the precipitate to form in the warm solution.

Part II. Filtering the Barium Sulfate Precipitate

1. 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.
2. 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 1).
   {12084_Procedure_Figure_1}
3. 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.
4. 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.
5. 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 2).
   {12084_Procedure_Figure_2}
6. 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 3).
   {12084_Procedure_Figure_3}
7. 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.
8. Rinse the beaker with small amounts of distilled or deionized water from the wash bottle and then transfer the washings to the filter.
9. 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.
10. Obtain a watch glass. Using a micro spatula, 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.
11. Using the micro spatula, 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.
12. 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.
13. Return the watch glass to the drying oven for an additional 5 minutes.
14. Remove the watch glass from the oven and set it aside to cool.
15. When cool, weigh the filter paper and the solid BaSO₄ on an analytical balance. Record the mass in the data table.
16. Repeat steps 13–15 until the mass readings do not change by more than 0.005 g.

Student Worksheet PDF

12084_Student1.pdf