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Determination of Waters of Hydration



Each and every chemical substance contains someamount of water molecules. The amount of water that is contained in achemical substance and can be removed through heating withoutchanging the chemical composition of the substance is referred to asthe water of hydration. In this laboratory experiment, the waters ofhydration for two unknown hydrate samples were determined.


This laboratory experiment aimed todetermine the water of hydration for different substances.

Background and Theory

The water molecules normallycombine with a substance in such a way that they can be removedthrough a heating process without distorting the chemical compositionof the substance. The amount of water that combines with a substanceand that can be eliminated through heating process withoutinterfering with the chemical composition of the substance isreferred to as the water of hydration (Clugston&amp Flemming, 2000).&nbspDuringheating, the substance is said to undergo dehydration.

Most of the ionic substances normally combine withammonia, water, and other polar molecules to form hydrates. Anexample of a hydrate is ZnSO4.7H2O,which is formed when water molecules combine with zinc sulphate.Normally, the hydrate of zinc sulphate is stable under normalconditions (Fifield &amp Kealey,2000).&nbspInthis laboratory experiment, hydrates of unknown compositions wereheated to eliminate the waters of hydration. A crucible and heaterwere used in the experiment. The percentage of the waters ofhydration for the two compounds were then determined. The waters ofhydration were determined by getting the difference between themasses of a covered crucible and sample and an empty crucible.

Safety Precaution

Safety is an important aspect in any laboratoryexperiment. In this laboratory experiment, proper eye protectivegears were put on to ensure that the eye was protected. Also, thecrucible was handled with care since it was being heated to hightemperatures. The crucibles were held with the use of tongs.

Materials and Methods

The materials used included a crucible, tongs,triangle holder, unknown hydrate samples, and Bunsen burner.

Experimental Procedure

The lab report was conducted in groups of twostudents each. Firstly, the porcelain crucible and cover were cleanedand dried. Secondly, the empty, covered crucible was then placed on atriangle holder and then heated until a cherry redness was formed.Thirdly, the crucible was allowed to cool down to the roomtemperature. After cooling, the weight of the crucible was thenmeasured to the nearest 0.001 grams. As the first crucible wascooling, the procedure was repeated with the second crucible.Fourthly, a sample of unknown hydrate was obtained from thelaboratory instructor. A small amount of the hydrate was then addedto the crucible and the total weighed of the covered crucible itscontent was measured and recorded as shown in the result section. Themeasurement was taken to the to the nearest 0.001 gram. The weightsof empty crucible and covered crucible with a hydrate were recordedin the data sheet. The covered crucible containing the hydrate samplewas placed the on the triangle holder and heated gently for someminutes. To minimize the loss of material from spattering during theinitial heating, the heating was done gently. The heating wascontinued for about fifteen minutes with hottest part of the Bunsenburner.

After thorough heating, the total weight of thecovered crucible and residue was measured and recorded in a datasheet. The crucible was then heated again for another five minutes.It was then allowed to cool and then reweighed. The process ofheating and weighing was repeated until two consecutive weights werethe same, with an error of 0.001 grams. As the first crucible withthe hydrate sample was cooling down, the entire process was repeatedwith a different unknown hydrate.

Experimental Results

The experimental results are as shown in the tablebelow.

Unknown Samples

Sample D

Sample C

1. Mass of crucible and cover

25.31 g

17.32 g

2. Mass of crucible, cover and the hydrate sample

26.25 g

18.36 g

3. Mass of hydrated compound= (2-1)

1.34 g

1.04 g

4. Mass of crucible, cover and sample after heating

25.96 g

17.89 g

5. Mass of anhydrous compound (4-1)

0.65 g

0.57 g

6. Mass of water = (3-5)

0.69 g

0.47 g

7. Percentage of water= (6/3 x 100%)

51.49 %

45.19 %

8. Moles of water=(6/molar mass H2O)

.0383 mol

.0261 mol

9. Molar mass of anhydrous compound

120 g/mol

161 g/mol

10. Moles of anhydrous compound= (5/9)

.00541 mol

.00354 mol

11. Waters of hydration(n)= (8/10)




As mentioned above, the water of hydration is defined as the waterthat is contained in substance, and that can be expelled through aheating process without distorting the chemical composition of thesubstance. It is good to note that, under normal environmentalconditions, virtually every substance contains some water molecules.The composition of the water of hydration changes with temperature.The water of hydration can also be lost as due to natural processessuch as evaporation (Klein, 2012). Inthis laboratory experiment, two unknown hydrates, C and D, were used.The hydrates were heated under identical environmental conditions. Inhydrate D, the percentage of water of hydration was found to be51.49, and that of hydrate D was found to be 45.19.

The water of hydration in a substance is determined as a percentageof the total mass of the substance. From the experimental resultsabove, it should be noted that different substances contain differentamount of water of hydration.


In this laboratory experiment, the waters of hydration for twohydrates, C and D, were determined successfully. It was found thatthe amount of water of hydration varies from one substance to theother. The experiment was successful as the main objective wasachieved.


Clugston, M. &amp Flemming, R.(2000).&nbspAdvancedchemistry&nbsp(1stEd.). Oxford: Oxford University Press.

Fifield, F. &amp Kealey, D. (2000).&nbspPrinciplesand practice of analytical chemistry&nbsp(1stEd.). Oxford: Blackwell Science.

Klein, D. (2012).&nbspOrganicchemistry&nbsp(1stEd.). Hoboken, N.J.: John Wiley.