Gravimetric Analysis Experiment

Summary

This video demonstrates the steps and data collection for a Gravimetric Analysis of an unknown sulfate. It covers the weighing of the sample, dissolving it in water and hydrochloric acid, and heating the mixture to form barium sulfate crystals. The procedure also includes filtration to isolate the precipitate, followed by heating the crucible to decompose the filter paper and recover the barium sulfate product. Key measurements, such as the mass of the beaker, sulfate, and crucible, are highlighted for accurate reporting. The process concludes with ensuring the final product is fully dried and its mass remains constant.

Takeaways

• 😀 Measure 0.3 to 0.4 g of the unknown sulfate, starting with taring the balance.
• 😀 Record the mass of the 250-mL beaker before adding the unknown sulfate.
• 😀 Dissolve the unknown sulfate by adding 50 mL of water and 20 drops of 6M hydrochloric acid.
• 😀 Heat the mixture just before boiling to form larger barium sulfate crystals.
• 😀 Measure 25 mL of 0.1 M barium chloride to add to the heated solution.
• 😀 The addition of barium chloride causes a cloudy mixture, indicating a precipitate formation.
• 😀 Let the precipitate settle for 20 minutes before moving to the next step.
• 😀 Preheat the crucible to remove contaminants, then weigh the empty crucible and record its mass.
• 😀 Filter the mixture using an ashless filter paper and funnel to separate the white precipitate.
• 😀 Dry the filter paper with the precipitate by heating the crucible in a fume hood until the paper decomposes.
• 😀 Reheat the crucible to ensure complete decomposition of the paper, then weigh the crucible again to confirm constant mass.

Q & A

What is the first step in the Gravimetric Analysis for the Unknown Sulfate lab?

The first step is to measure out 0.3 to 0.4 grams of the unknown sulfate from a small vial, making sure to tare the balance before weighing.

Why is it important to tare the balance before weighing the unknown sulfate?

Taring the balance ensures that it starts at zero, so that only the mass of the unknown sulfate is measured, excluding the mass of the beaker.

How is the unknown sulfate dissolved in the lab procedure?

The unknown sulfate is dissolved by adding 50 mL of water and 20 drops of 6M hydrochloric acid to the beaker containing the sulfate.

What is the purpose of heating the mixture in the lab procedure?

The mixture is heated to just before boiling to help form larger crystals of barium sulfate, which is the desired product of the reaction.

How is barium chloride added to the solution in the lab?

25 mL of 0.1 M barium chloride is measured and slowly poured into the hot solution through a stirring rod, causing a cloudy precipitate to form.

What does the cloudiness in the solution indicate?

The cloudiness signifies that a precipitate, specifically barium sulfate, has formed in the solution.

What is the purpose of filtering the mixture?

The mixture is filtered to separate the solid barium sulfate precipitate from the liquid, ensuring only the solid is collected for further processing.

How is the filter paper prepared for the filtration step?

The ashless filter paper is folded into a cone shape and moistened with water to help it adhere to the funnel during filtration.

What happens during the heating of the crucible containing the filter paper and precipitate?

The crucible is heated to decompose the filter paper, turning it black as it chars, and eventually all the paper decomposes to carbon dioxide, leaving only the white barium sulfate precipitate.

Why is it necessary to reheat the crucible after the initial weighing?

Reheating the crucible ensures that all the filter paper is fully decomposed, and any remaining moisture is removed, allowing for an accurate final mass measurement.