PURPOSE To determine the amount of water of crystallization in one mole of a hydrated salt and its formula

                                CuSO4.5H2O                 CuSO4 + 5H2O

BACKGROUND

The amount of water present in the hydrate is determined by heating the hydrated salt in a crucible and thereby measuring the difference between the mass of the salt before and after heating.

APPARATUS

1 Ceramic crucible

1 Bunsen burner

1 Ring stand

1 Pipe -clay triangle

1 Tongs

1 Electronic balance (+ 0.01g)

REAGENTS

Hydrated copper sulphate

SAFETY

1. Hot items look the same as cool items. Be sure to wait until glassware is cool before transferring it from place to place
2. Wear laboratory safety goggles and a lab apron at all times in the laboratory.

VARIABLES

• Independent: mass of hydrated copper sulphate
• Dependant: percentage composition of water in the hydrated copper sulphate
• Control: room temperature,         

PROCEDURE

1. Weighed a clean, empty crucible with its lid and record the mass.
2. With the crucible still on the balance, added approximately 2 grams of the hydrated salt to the crucible. Then I recorded the mass.
3. Heated the crucible gently on a burner for 3-5 minutes and covered it. Was careful not to allow the crucible to turn red, as overheating decomposes the sample.
4. Turned off the burner.
5. Reweighed it when the crucible has cooled
6. Gently heated the crucible for 3 more minutes, allowed to cool and reweighed. If the mass was the same as the first weighing, I stopped. If the mass had changed, continued gently heating and cooling until the mass does not change.
7. Repeated with 2 more samples.

DATA COLLECTION AND PROCESSING:+

 

 

CALCULATIONS:

1. The value of x in  CuSO4.xH2O

Sample 1:

Mass of water = mass of hydrated salt – mass of anhydrous salt

                   = 2 – 1.28

                   = 0.72 g

Moles of water = mass / Mr

                = 0.72 / 18

                =0.04

Moles of CuSO4 = mass / Mr

                   = 1.28 / 159.62

                   = 0.008

Moles of hydration = moles (water) / moles (CuSO4)

                        = 0.04 / 0.008

                        = 5

Sample 2:

Mass of water = mass of hydrated salt – mass of anhydrous salt

                   = 3 – 1.9

                   = 1.1 g

Moles of water = mass / Mr

                = 1.1 / 18

                =0.06

Moles of CuSO4 = mass / Mr

                   = 1.9 / 159.62

                   = 0.011

Moles of hydration = moles (water) / moles (CuSO4)

                        = 0.06 / 0.012

                        = 5

Sample 3:

Mass of water = mass of hydrated salt – mass of anhydrous salt

                   = 4 – 2.6

                   = 1.4 g

Moles of water = mass / Mr

                = 1.4 / 18

                =0.08

Moles of CuSO4 = mass / Mr

                   = 2.6 / 159.62

                   = 0.016

Moles of hydration = moles (water) / moles (CuSO4)

                        = 0.08 / 0.016

                        = 5

2. Percentage composition of water

Molar mass (CuSO4.5H2O) = 63.55 + 32.06 + (16.00*4) + (10*1) + (5*16.00)

                                 = 249.61 g mol-1

Molar mass (5H2O) = 90.0 g mol-1

%  H2O = (90.0 g mol-1 ) / (249.61 g mol-1) * 100

        =36.1%        

CONCLUSION:

 

My results matched with my initial hypothesis that copper sulphate has 5 moles of water in its hydrated form. I further found out that that the water of crystallization is responsible for the shape of the crystals. In fact the arrangement and the shape of the crystal can be determined by the science of x-ray crystallography. Crystallographic methods now depend on the analysis of the  patterns of a sample targeted by a beam of some type. Sometimes the last water is a bit hard to drive off. Crushing the crystals will give you a lot more surface area. Weigh the powder, not the crystals. In a hydrate the water molecules are a distinct part of the compound but are joined to the salt by connections that are weaker than the connections in the salt or the connections in the water molecules. We use a dot to connect the water units to the salt formula ( CuSO4.5H2O). Hence the name of the salt is copper sulphate pent hydrate. It’s used as an herbicide, fungicide and pesticide and mainly for chemistry education in schools.

                                                

EVALUATION:

The dish must be weighed before the compound absorbs any moisture from the air and so should have covered it with a filter paper. Also I did not weigh the mass of the crucible and its contents when it is hot as heat waves tend to spiral upwards, resulting in masses to be measured less than the actual value. All these factors do affect the results. Sometimes the last water is a bit hard to drive off. Crushing the crystals   will give a lot more surface area. Hence, one should weigh the powder, not the crystals.                         

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