Determining the water of crystalisation

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II LO im. Gen. Zamojskiej i Heleny Modrzejewskiej Poznań

IB School no. 1349

DETERMINING THE WATER OF CRYSTALISATION

INTRODUCTION

Water of crystallization is water, which is present in the compound but is not covalently bonded to the host molecule. Hydrated salt (also known as hydrate) is an ionic compound, which is associated with water of crystallization. To show how much the salt is hydrated the notation can be written: salt  nH2O; where n represents the number of water molecules per one molecule of salt.

The aim of this investigation is to find the number of moles of water present in hydrated salt: copper sulphate (CuSO4).

Equipment, materials and reagents.

Variables.

Environmental care and safety.

The product is obtained in a small amount and does not make up a potential danger for the environment, thus it can be disposed down the sink.

Hydrated copper sulphate is poisonous and it can be absorbed into the body by inhalation or by ingestion. Moreover it can irritate the eyes and skin so apron, glasses and protective gloves are required. It is also advised to make the investigation in well-ventilated room to avoid the inhalation. The tongs should be used for carrying the hot crucible, and the special care should be paid while using the Bunsen burner.

Procedure of the experiment.

To measure the water of crystallisation of salt (CuSO4)  I need to measure the mass of hydrated salt, and its mass after dehydration. Therefore I can calculate the mass of water in hydrated salt.

Using weight balance I measured the mass of crucible with lid and recorded the date. Then I put the certain amount of CuSO4 to the crucible and recorded its mass. I put the crucible with lit and salt onto Bunsen burner and heated it strongly for 4 minutes. At the end I put off the lit to let the water moisture to evaporate. Since the mass of crucible can differ in different temperatures I had to leave it to cool down (with lid on it to avoid the water from the air to be gained by the salt). When crucible obtained room temperature I put it on weight balance and recorded its mass. Using Bunsen burner I heated my crucible for another 5 minutes and at the end took off the lid. I recorded the mass of crucible with salt after it cooled down. I had to repeat these activities since the mass of crucible with lid and salt was stable. I had to do that because only by this I could be sure that the whole water from the salt had evaporated.

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Fig 1. Heating of the salt in the crucible with lid

Procedure of analyzing the data collected.

In order to calculate the number of moles of water present in hydrated copper sulphate (CuSO4) I made the calculations presented below:

Firstly I calculated the mass of anhydrous salt and the mass of water, which evaporated by subtracting the mass of anhydrate salt from the mass of hydrated salt. Then I calculated the molar masses of anhydrate salt and water. After ...

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