Web Percentage Water in a Hydrate - Palomar College

[Pages:3]PERCENTAGE (%) OF WATER IN A HYDRATE

OBJECTIVE: The objective of this experiment is to determine the % of water in a HYDRATE.

MATERIALS: Barium chloride dihydrate, crucible and cover, tongs, clay triangle, ceramic sheet, balance.

DISCUSSION:

Many inorganic salts form crystalline compounds that contain water molecules (water of hydration) as part of their crystalline structure.

Examples: CaSO4 ? 2H20 , CoCl2 ? 6H20 , CuSO4 ? 5H2O

In a hydrate, water molecules are distinct parts of the compound but are joined to it by bonds that are weaker than either those forming the anhydrous salt (without water) or those forming the water molecules. In the formula of a hydrate, a dot is commonly used to separate the formula of the anhydrous salt from the number of water molecules.

Hydrates can usually be converted to the anhydrous form by heating:

Hydrated salt --- - anhydrous salt + water

= heat

CuSO4 ? 5H20 ---- CuSO4

+ 5H20

Hence it is possible to determine the percentage of water present in a hydrated salt by determining the amount of mass lost in heating.

% water = mass of water x 100 mass of hydrate

PROCEDURE : WEAR PROTECTIVE GLASSES!! 1. Place a clean crucible and cover on a clay triangle and heat strongly (for 5 minutes to drive off any

volatile material) using the laboratory burner.

2. Using crucible tongs, move the crucible and cover to a ceramic pad to cool down.

3. After they have cooled down to room temperature (about 10 minutes) weigh them accurately. (Never place a hot object on the balance pan.

4. Place about half gram of barium chloride dihydrate in the crucible and determine the mass with cover.

5. Heat the crucible with contents moderately for 20 minutes. Adjust the cover so that it is slightly ajar to allow the water vapor to escape (see figure). The crucible bottom should be pale orange during heating.

6. Allow to cool down again and determine the mass.

7. Calculate the % of water in your sample.

CHEMISTRY 100L

NAME__________________________ WEIGHING/DEHYDRATION EXERCISE

DATA

Mass of Crucible and Cover

____________________

Mass of Crucible, Cover, and Hydrate

____________________

Mass of Crucible, Cover, and Anhydrous Salt ____________________

CALCULATIONS (Show Set Ups): 1. Calculate the mass of hydrate.

2. Calculate the mass of water.

3. Calculate the percent water in the hydrate.

4. From the formula BaCl2 ? 2H20, calculate the theoretical percent water in the hydrate. 5. Calculate the percent error.

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