Finding the number of water molecules attached to copper sulphate crystals CuSO4.xH2O.
* To avoid error:
1. Do not make too much movement around the balance when taking any mass.
2. Wait till the balance’s reading is stable then take the reading.
3. Make sure that the crystals are well distributed in the beaker to be evenly exposed to as much heat as possible.
* In case the experiment was repeated for more accuracy in the final results, variables should be controlled:
1. Use the same balance.
2. Use same size and shape of the container (Beaker or evaporating dish)
3. Use exactly the same mass of copper sulphate crystals.
4. Apply the same amount of heat at the same time intervals.
5. Control room temperature.
3. Watching glass.
4. Glass stirring rod.
5. Hot plate.
1. The mass of the empty beaker is measured using the balance and found to be 29.92g
2. The mass of the watching glass is measured using the balance and found to be 58.95g
3. The copper sulphate, which is a light blue powder, is then added to the beaker and the total mass of the beaker and the copper sulphate is 33.86g
4. Wear the goggles for safety.
5. Copper sulphate is heated using the hot plate, the color of the powder gradually changes from light blue to white starting from the bottom of the beaker up and water vapor can be seen on the beaker’s walls.
6. The powder becomes a little clumpy during heating.
7. The beaker is shaken and the powder is stirred using the stirring glass rod every now and then to expose the upper parts of the powder to more heat and allow them to dehydrate properly.
8. As soon as the entire powder turns white, the beaker is covered with watching glass to prevent copper sulphate from reacting, again, with water vapor in the atmosphere. Also, the hot plate is now turned off.
9. The mass of the dehydrated copper sulphate along with that of the beaker and the watching glass is now taken and found to be 91.60g.
1. By subtracting the mass of the empty beaker measured at the beginning of the procedure from its mass along with the copper sulphate, the mass of hydrated copper sulphate used at the beginning is found.
33.86g – 29.92g = 3.94g
2. By subtracting the mass of the empty beaker and watching glass measured at the beginning of the procedure from the final mass of the procedure, the mass of dehydrated copper sulphate is obtained.
91.60g – (58.95g + 29.92g) = 2.73g
3. The mass of evaporated water molecules can now be found by subtracting the mass of dehydrated copper sulphate from the mass of hydrated copper sulphate.
3.94g – 2.73g = 1.21g
4. The number of moles of evaporated water is now calculated by the rule:
Number of moles = Mass / Molar mass
Where the molar mass of water is 18 g/mol
n = 1.21g / 18g.mol-1
= 0.067 mol
5. The number of water molecules is now calculated by multiplying the number of moles times Avogadro’s number (6.02*1023)
Number of molecules = n*NA
6. The ratio is calculated to find the number of molecules attached and the formula of the compound:
Molar mass (g/mol)
Number of moles (mol)
3.9 ï¿½ 4
The aim of this was to determine the number of water molecules attached to copper sulphate and that is a 4.03*1022 molecule where the compound has a formula of 10CuSo4.4H2O
* Sources of error:
1. Uncertainty of the balance used.
2. Stirring may cause crystals to stick on the stirring glass rod affecting the mass.
3. The beaker has a small surface area, therefore not all particles will be exposed to enough heat and so not all water molecules will evaporate.
* Ways of improving:
Use a container with a larger surface area such as an evaporating dish to avoid the need of stirring and therefore copper sulphate crystals won’t be lost by sticking to the glass rod. Also, this will increase the number of water molecules exposed to heat and so it will increase the chance of dehydrating all of the copper sulphate crystals.