UNIT 2 Percentage of Water in a Hydrate – INORGANIC CHEMISTRY PRACTICALS

UNIT 2 Percentage of Water in a Hydrate

1.0 Introduction

2.0 Objective

1. Main Content
2. Theory
3. Procedure

4.0 Conclusion

5.0 Summary

6.0 Tutor Marked Assignments

7.0 References/Further Reading

1.0 Introduction

Many pure substances combine with water in a fixed mole ratio to yield compounds called hydrates. For example, zinc sulfate combines with water to form crystalline ZnSO₄.7H₂O which is a stable compound at normal atmospheric conditions. All pure samples of this hydrate show the same percentage of water by analysis. Thus, this hydrated compound obeys the law of constant composition. Upon heating a sample of such a hydrate, it may lose all its water of hydration and revert to the anhydrous salt. Substances which have adsorbed water on the surface do not show constant composition and therefore are not hydrates. An example of this would be common table salt, which becomes very sticky on humid when the relative humidity is high. In these cases, the percentage of water is not constant for all samples of a particular compound, and the water is not chemically bonded as part of the crystal structure.

2.0 Objective

By the end of this unit, you shall be able

• to explain the concept of water of hydration
• perform an experiment to determine the percentage of water in an unknown hydrate.
  
  Define the terms : efflorescence , deliquescence and hygroscopy

3.0 Main Content

3.1Theory Hydrates are crystalline salts that are bonded to water molecules in definite proportions. The weakly bound water is known as either the water of hydration or water of crystallization. The fixed numbers of water molecules that are weakly bonded to the salt are represented as follows: salt • number of waters zinc sulfate heptahydrate

(ZnSO4 • 7 H2O)

The dot represents the weak salt • water bond in the chemical formula. The bond is so weak that simply heating the hydrated salt to liberate the water molecules as vapor can normally break it. If water is added to the now anhydrous salt, the reverse will take place with the waters reattaching themselves to the salt. This is known as a reversible action.

There are three closely related substances that act similar to hydrates but have distinct individual characteristics. These are hydroscopic, deliquescent, and efflorescent substances.

Hydroscopic substances – readily absorb moisture from the air and are used as drying agents (desiccants).

Hygroscopy is the ability of a substance to attract and hold water
molecules
from the surrounding environment

Deliquescent substances – continue to absorb water from the air until they form a solution. Deliquescence the process in which a soluble
substance picks up water vapor
from the air to form a solution.

Efflorescent substances – are hydrates that lose water when simply exposed to the atmosphere. Efflorescence (which means “to flower out” in French) is the loss of water (or a solvent) of crystallization
from a hydrated
or solvated salt
to the atmosphere
on exposure to air.

As previously mentioned, each hydrated salt has water molecules bonded to them in definite proportions. The percent water in the hydrated salt can be calculated, theoretically, using the chemical formula of the hydrate.

% H

3.2 Procedure

Clean and dry a porcelain crucible and cover. Place the empty, covered crucible on a clay triangle supported by a ring on a ring stand. Heat the crucible and cover in the hottest flame of the Bunsen burner for 5 minutes. A dull red glow should be observed on the crucible and cover. This will require that you manipulate the burner to heat uniformly. This will ensured that all volatiles and combustible materials are removed prior to the analysis procedure and that a constant weight for the crucible and cover may be recorded. The crucible and cover must be allowed to cool to laboratory temperature for approximately 15 minutes. Using crucible-tongs transfer the crucible and cover to a carrying tray and weigh them to the nearest 0.001 g. Add 1 to

1.5 g of zinc sulfate heptahydrate to the crucible and weigh the covered crucible to the nearest

0.001g.

Place the covered crucible on the clay triangle with the cover slightly opened. Heat the crucible gently for a few minutes to avoid loss
of material from spattering during initial heating.

Remember contents for approximately 15 minutes gradually increasing the temperature of the flame. Allow the crucible to cool on the triangle after removing the flame until it has reached room temperature.

Transfer it to the carrying tray and weigh the covered crucible to the nearest 0.001 g. Reheat the crucible and contents for about 5 minutes and, after cooling, weigh it again. Repeat this heating, cooling and weighing sequence until two consecutive weighings are within 0.005 g. This sequence of heating, cooling and weighing to obtain consistent results is known as heating to
a constant weight.

That water boils a t 100^oC. Continue to heat the crucible and calculate the actual percentage of water in the zinc sulfate using the gram formula weight of the hydrate and the weight of water indicated in the formula multiplied by 100. Calculate the experimental percentage of water in your hydrate based on water lost as a result of heating divided by the hydrate sample weight multiplied by 100. To evaluate your results and determine how well you did the analysis, you can calculate relative error using the following formula.

Percent of Relative Error

An absolute relative error of 3% or less is excellent. If the percent of error is greater than 3%, think about factors that you observed during the procedure that may have affected your results Modify your actions accordingly for the unknown hydrate sample. Obtain from your instructor an unknown hydrate sample. Repeat the procedure which you used on the known hydrate. Do a minimum of two trials and report the average percentage of water in your unknown hydrate. DO NOT forget to record your sample number and/or letters.

Requirements for Report

Data tables should be constructed for all hydrate trials so the information is readily found in the report. One sample calculation must be demonstrated for a completed trial. Record all observations and adjustments made to improve your technique.

4.0 Conclusion Water that is chemically combined with other molecules (salt) to form a hydrate mineral that is easily removed through heating.

5.0 Summary

In this unit you have been through the concept of water of hydration and you have also performed an experiment to determine the water of hydration of a salt.

6.0 Tutor Marked Assignments (TMA)

1. Using a suitable reference source, define the following words:

• Deliquescence
• Efflorescence
• Hygroscopy

1. Anhydrous calcium chloride is used inside desiccators to remove moisture. Explain how CaC1₂ removes the water vapor.
2. Calcium chloride hexahydrate has the chemical formula CaCl₂ • 6 H₂O. What is the theoretical percentage of water?

Solution

The formula weight of CaCl2 • 6 H2O is:

Ca: 1 x 40.1 = 40.1 amu

Cl: 2 x 35.5 = 71.0 amu

H: 12 x 1.0 = 12.0 amu

O: 6 x 16.0 = 96.0 amu

Formula weight of CaCl2 • 6 H2O = 219.1 amu Formula weight of H2O = 18 amu

% H₂ O =