Gravimetric Analysis of a Chloride Salt

Summary

In this lab experiment, participants engage in gravimetric analysis of unknown chloride salts. They begin by weighing a salt sample, dissolving it in water with nitric acid, and adding silver nitrate to precipitate silver chloride. The process involves careful heating to coagulate the precipitate and filtering it using vacuum filtration. Participants must decant the supernatant, rinse the precipitate, and test for excess silver ions. Finally, the precipitate is dried and weighed, with precise mass readings crucial for determining the chloride percentage in the sample.

Takeaways

• 😀 Start by selecting an unknown chloride salt sample and recording its sample number.
• 📏 Use analytical balances to weigh out a sample between 0.1 and 0.15 grams.
• 💧 Dissolve the salt in approximately 100 mL of water, adding 1 mL of 6 molar nitric acid for complete dissolution.
• 🧪 Calculate the necessary amount of 0.1 molar silver nitrate to precipitate the chloride ions from the solution.
• ❄️ Heat the mixture gently to encourage precipitation while avoiding boiling and excessive stirring.
• 🔬 Set up vacuum filtration to separate the formed precipitate from the liquid solution.
• 💦 Rinse the precipitate with dilute nitric acid to prevent re-dispersion of small particles.
• ⚗️ Use a rubber policeman to transfer the precipitate onto the filter efficiently.
• 🔍 Test the filtrate for excess silver ions by adding hydrochloric acid and observing for any precipitate.
• 🔥 Dry the precipitate in an oven, noting the temperature and duration to ensure accurate mass measurement.

Q & A

What is the primary purpose of the gravimetric analysis experiment discussed in the transcript?

The primary purpose is to determine the percentage of chloride by mass in unknown salt samples.

How do participants prepare their salt sample for the experiment?

Participants weigh out a salt sample using analytical balances, aiming for a mass between 0.1 grams and 0.15 grams, and then dissolve it in about 100 mL of water.

Why is nitric acid added to the dissolved salt sample?

Nitric acid is added to ensure complete dissolution of the salt sample before proceeding with the precipitation of chloride ions.

What is the significance of adding silver nitrate to the solution?

Silver nitrate is added to precipitate chloride ions as silver chloride, which is minimally soluble in water.

What should be done if the supernatant liquid remains cloudy after heating?

If the supernatant is still cloudy, continue heating gently until it becomes clear, ensuring that enough silver nitrate has been added to precipitate all chloride ions.

What is the function of the desiccator in this experiment?

The desiccator is used to maintain a dry environment for the crucibles before weighing the final precipitate to avoid moisture interference.

How do participants ensure that excess silver is removed from the precipitate?

Participants rinse the precipitate with dilute nitric acid and test the washings with hydrochloric acid to check for the formation of any additional precipitate, indicating excess silver.

What is the role of the rubber policeman in the filtration process?

The rubber policeman helps transfer the precipitate from the beaker to the filter, ensuring that all solid material is collected.

What criteria must be met for the mass of the dried silver chloride to be considered valid?

Two successive mass readings of the silver chloride should not differ by more than one milligram to confirm that it is fully dried.

What precautions should be taken while handling the salt during the experiment?

Participants should avoid exposing the salt to light, as it is photosensitive and can decompose, and handle all glassware carefully to maintain the integrity of the samples.