PERFORMANCE ASSIGNMENT                          Lab Eight

The Formula of a Hydrate

Many salts when crystallized appear to be dry yet when heated yield large amount of water. such compounds are called hydrates. The number of moles of water per mole of anhydrous salt is usually a simple number. The dot between the salt and water does not mean multiplication but that the water molecules are loosely attached to the other atoms.

Objectives

1. Determine the number of moles of water evaporated.
2. Determine the molar ratio between salt and water.
3. Identify the correct hydrate.

Procedure

1. Place a clean crucible with the cover askew on an iron ring. heat with a bunsen burner flame for two or three minutes to clean and dry the crucible.
2. After about three minutes use tongs to transfer the cool crucible and cover to the balance and weight.
3. Fill the crucible with about one-fourth full of the hydrate crystal. weight the crucible, over, and contents.
4. Place the crucible, cover askew and heat gently to avoid spattering. Increase the flame until the botttom is a dull red. Maintain this for five minutes. Transfer the crucible to the cesiccator with a pair of tongs.
5. Wait aboutfive minutes and transfer the crucible to the balance and weigh.
6. Reapeat this again and weigh. If the results do not agree within 0.03 grams repeat.

Data

mass of crucible and cover
mass of crucible, cover and hydrate
mass of crucible, cover, and salt after heating
mass of crucible, cover, and salt after second heating
mass of 1 mole anhydrous salt

Calculations

1. Calculate the number of moles of the anhydrous salt prepared:

2. Calculate the number of moles of water driven from the sample of hydrate

3. Calculate the moles of water per mole of anhydrous salt.

4. Write the empirical formula for the hydrate.

5. Determine the correct hydrate used.

---

Back to Syllabus

Back to Laboratories

Scoring