CHEMLAB Analyzing a Mixture

Chemists often analyze mixtures to determine their compositions. Their analytical procedures may include gas chromatography, mass spectrometry, or infrared spectroscopy. In this ChemLab, you will use a double-displacement reaction between strontium chloride and sodium sulfate to analyze a mixture of sodium sulfate and sodium chloride.

PROBLEM

What is the mass percent of sodium sulfate in a mixture of sodium sulfate and sodium chloride?

OBJECTIVES

• Observe the double-displacement reaction between strontium chloride and sodium sulfate.

• Quantify the amount of strontium sulfate produced.

• Compare the mass of strontium sulfate produced with the mass of sodium sulfate that reacted.

Hypothesis:

MATERIALS

funnel

wash bottle with distilled water

filter paper

250-mL beakers (2)

50-mL graduated cylinder

stirring rod

ring stand

iron ring

spatula

weighing dish or paper

balance

SAFETY PRECAUTIONS

Advise students to wear safety goggles and an apron.

PROCEDURE

1. Weigh the weighing dish or weighing paper and record its mass in the data table where indicated.

2. Transfer a 0.500- to 0.600-g sample of the sodium sulfate-sodium chloride mixture to the dish or paper and weigh it.

Record the total mass of the sample and container in the data table.

3. Transfer all of the mixture to a 250-mL beaker, add approximately 50 mL of distilled water, and stir slowly until the sample is completely dissolved.

4. Use a graduated cylinder to measure 15 mL of 0.500M strontium chloride solution, then pour it into the solution in the beaker. Stir slowly for 30 s to completely precipitate the strontium sulfate.

5. Obtain a piece of filter paper, fold it in half, and tear off one corner. Fold the paper in half again. Weigh the folded paper and record its mass in the data table where shown.

6. Clamp a funnel support or an iron ring to a ring stand, put the funnel in the support or ring, insert the folded filter paper in the funnel, and wet the paper with a small amount of distilled water from your wash bottle. Set an empty 250- mL beaker under the funnel to receive the filtrate.

7. Carefully transfer the mixture, including all of the precipitate, from the beaker to the filter paper, using a stirring rod and a wash bottle as demonstrated by your teacher.

8. Rinse the precipitate by pouring about 10 mL of distilled water into the filter paper.

9. When liquid no longer drips from the funnel, carefully remove the filter paper and residue from the funnel. Unfold the paper and spread it on a paper towel to dry overnight. Write your name on the paper towel.

10. Dispose of the filtrate according to your teacher’s instructions.

11. Weigh the dry filter paper the following day, and record the mass of filter paper with residue in your data table.

DATA AND OBSERVATIONS

ANALYZE AND CONCLUDE

1. Interpreting Data What is the balanced equation for the reaction between sodium sulfate and strontium chloride in aqueous solution?

2. Making Inferences After the reaction, does the sodium chloride produced and the unreacted sodium chloride from the original sample appear in the precipitate or the filtrate?

3. Interpreting Data What mass of strontium sulfate is produced in the reaction? Subtract the mass of filter

paper from the mass of strontium sulfate plus filter paper.

4. Interpreting Data What is the mass and mass percent of sodium sulfate in the original sample?

APPLY AND ASSESS

1. Could you precipitate the sulfate from the sodium sulfate in the sample by adding an excess of aluminum chloride solution? Explain.

2. If your strontium sulfate precipitate was not completely dry when you weighed it, how would it affect your value of the mass percent of sodium sulfate in the sample?

3. How could you have tested the precipitate to be sure it contained strontium?