Mass Determination – Water of Crystallization

Aim: To determine the water of crystallization of a hydrated salt.

Hypothesis: Sodium carbonate exists as a hydrated salt where 10 mols of H2O are bonded to 1 mol of Na2CO3.

Apparatus and Materials:

Test tubes

Analytical balance

Clamp

Bunsen burner

Heat insulated gloves

Weighing dishes

Spatula

5 grams hydrated salt (sodium carbonate decahydrate)

Theory:

A hydrate salt is composed of anions (negative ions) and cations (positive ions) which are surrounded by and weakly to bonded water molecules. Each hydrate salt has a fixed number of water molecules associated with it, called waters of hydration or water of crystallization. When a salt holds waters of hydration, we call it a hydrated salt or a hydrate.

Sodium carbonate decahydrate, Na2CO3•10H2O, has ten waters of crystallization, or ten waters of hydration. Other hydrates have waters of hydration ranging from one to twelve. Upon heating, a hydrate decomposes and produces an anhydrous salt and water (in the form of steam).

Na2CO3•10H2O (s) → Na2CO3 (s) + 10 H2O (g)

Procedure:

The weight of the dry empty test tube (W1) was first found and recorded.

The test tube with approximately 5 grams of the hydrated salt added was then reweighed (W2).

With the use of a test tube clamp, the contents of the test tube were heated over a Bunsen burner.

The heat was gradually increased and this process was continued for 15 minutes.

The hot test tube was then allowed to cool for 10 to 15 minutes.

The test tube and its contents were then weighed (W3).

All weights, times, and observations during the experiment were then recorded.

Results:

Table Showing Masses Obtained During Experiment

Weight of Empty Test Tube, W1 (g) 32.23

Weight of Test Tube and Hydrated Salt, W2 (g) 37.20

Weight of Test Tube and dehydrated Salt, W3 (g) 34.08

Calculations:

Mass of Sodium Carbonate (W3-W1) = 1.85g

Mass of water given...