PERCENTAGE OF WATER IN A HYDRATE

Objectives

1. To determine the percentage of water in barium chloride dihydrate.

2. To determine the percentage of water in an unknown hydrate salt.

3. To calculate the water of crystallization for the unknown hydrate salt.

Discussion

A hydrate salt is composed of anions (negative ions) and cations (positive ions) which are surrounded by and weakly 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 (hydrate from hydor, the Greek word for water).

Barium chloride dihydrate, BaCl2(2H2O, has two waters of crystallization, or two 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).

BaCl2(2H2O (s ) ( BaCl2 (s) + 2 H2O (g)

Examples and Problems

Problem 1-Theoretical Percentage

Calculate the theoretical percentage of water in barium chloride dihydrate.

Solution: The formula mass of BaCl2(2H2O is

Ba1 x 137.3 = 137.3 amu

Cl2 x 35.5 = 71.0 amu

H2O2 x 18.0 = 36.0 amu

244.3 amu

The theoretical percentage of water is found by dividing the water of crystallization mass, 36.0 amu, by the hydrate mass, 244.3 amu.

36.0 amu x 100 = 14.7 %

244.3 amu

Problem 2 – Experimental percentage

The experimental percentage of water in a hydrate is found by comparing the mass of water driven off to the total mass of the compound, expressed as a percentage.

A 1.250 g sample of barium chloride dihydrate has a mass of 1.060 g after heating. Calculate the experimental percentage of water.

Solution:The mass of water lost is found by difference:

1.250 g – 1.060 g = 0.190 g

hydrate anhydrous water

salt

The experimental percentage of water is

mass of water x 100 = % of water

mass of hydrate

0.190 g x 100 = 15.2% water

1.250 g

Problem 3 – Water of crystallization

Calculate the water of crystallization for an unknown hydrate that is found to contain 30.6 % water. The formula mass of the anhydrous salt (AS) is 245 amu.

Solution: The unknown hydrate is 30.6 % water. subtracting from 100 %, the hydrate must be 69.4 % anhydrous salt. If we make the sample 100 g, the mass of water is 30.6 g and the anhydrous salt 69.4 g. to calculate the moles of water and the anhydrous salt (AS)

30.6 g H2O x 1 mole H2O = 1.70 moles H2O

18.0 g H2O

69.4 g AS x 1 mole AS = 0.283 mole AS

245 g AS

To find the water of crystallization, simply divide the mole ratio of water to anhydrous salt.

1.70 moles H2O = 6.01 ( 6

0.283 mole AS

The water of crystallization is always a whole number, therefor the formula for the unknown hydrate is AS(6H2O.

PROCEDURE

In this lab, you will find the experimental percentage of water in BaCl2(2H2O, you will also find the percentage of water in an unknown hydrate and then calculate the water of crystallization of the unknown (the formula mass will be provided).

A. Percentage of water in Barium Chloride dihydrate.

1. Obtain a porcelain crucible from the stockroom, rinse with water, and heat on a clay triangle for five minutes over a direct flame to remove any surface moisture. Use crucible tongs when handling hot crucibles.

2. Weigh the cool, dry crucible and lid. Record the weight on the data sheet.

3. Place between 1.0 and 1.5 g of BaCl2(2H2O in the crucible and reweigh the crucible, lid and contents. Record the mass on the data sheet. Calculate the weight of the hydrate. Record the mass of the hydrate on the data sheet.

4. Place the crucible back on the clay triangle with the lid almost...

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