You placed a sample of a hydrate of calcium chloride (CaCl2) in a weighed test tube, and weighed the filled test tube.

Then you heated it until no more water was evolved. After cooling, you weighed the test tube again.

Mass of empty tube (g) 13.5

Mass of filled tube before heating (g) 18.5

Mass after cooling (g) 16.2

Calculate the following:

1. Mass of hydrate originally taken

2. Mass of water evolved

3. Mass of anhydrous CaCl2 formed

4. Moles of anhydrous CaCl2

5. Moles of water evolved

6. Moles of water in the molecular formula of the hydrate (give your answer to 2 decimal places)

1. 5g

2. 2.3g

3. 2.7g

4. 0.02mol

5. 0.13mol

6. 7moles

Explanation:

From the question, the following were obtained:

Mass of empty tube = 13.5g

Mass of empty tube + hydrated salt = 18.5g

Mass of tube + anhydrous salt = 16.2g

1. Mass of empty tube = 13.5g

Mass of empty tube + hydrated salt = 18.5g

Mass of hydrated salt = 18.5 - Mass of empty tube

Mass of hydrated salt = 18.5 - 13.5 = 5g

2. Let us calculate the mass of the anhydrous salt.

Mass of tube + anhydrous salt = 16.2g

Mass of empty tube = 13.5g

Mass of anhydrous salt = 16.2 - Mass of empty tube = 16.2 - 13.5

Mass of anhydrous salt = 2.7g

Now we can calculate the mass of the water evolved as follows:

Mass of water = Mass of hydrated salt - Mass of anhydrous

Mass of water = 5 - 2.7 = 2.3g

3. Mass of empty tube = 13.5g

Mass of anhydrous salt = 16.2 - Mass of empty tube = 16.2 - 13.5

Mass of anhydrous salt = 2.7g

4. MM of CaCl2 = 40 + (2x35.5) = 40 + 71 = 111g

Mass of CaCl2 = 2.7g

Number of mole = Mass / Molar Mass

Number of mole of CaCl2 = 2.7/111 = 0.02mol

5. MM of H2O = (2x1) + 16 = 2 + 16 = 18g/mol

Mass of H2O = 2.3g

Number of mole = Mass / Molar Mass

Number of mole of H20 = 2.3/18 = 0.13mol

6. To get the mole of water in the molecular formula, we will find the ratio of the number of mole of anhydrous salt to water as shown below:

Mole anhydrous : mole of water ie

0.02 : 0.13 = 1 : 7

Therefore, the mole of water in the formula is 7 ie

CaCl2.7H20