## Calendar

| Friday, 19 June 2015 |

12:00 AM

### June 2016

Instructions

For this exam, you will be required to conduct and experiment, and submit a report. You will be given one hour to collect your data, and then a further hour to write your report. At the end of this time, you will hand in your report to the teacher.

In order to undo their bad reputation, local mad scientists have decided to start donating their unused chemicals to local schools. One of the chemicals that was donated to Wynberg Boys' High was a big bottle of copper sulphate (CuSO4.5H2O). However, they are not necessarily to be trusted, and we're worried that they have given us an impure sample.

The Science department at WBHS has asked you to conduct an experiment to determine if the sample of powder is pure or not. If it is not pure copper sulphate, any experiments performed could be very dangerous.

You are given the following information:

CuSO4.5H2O is a blue crystal. This is a form of copper sulphate that has water molecules trapped in its crystal structure. The 5H2O at the end of the formula tells you that there are always 5 H2O molecules for every CuSO4These can be removed by heating the crystals in a test tube until all of the water has evaporated. As the water is removed, the crystals change colour from blue to white, as seen in the picture below.

• In the sample on the left, the water is still present.
• In the sample on the right, the water has been removed by heating.

In a pure sample of CuSO4.5H2O, the water molecules account for 36,07% of the total mass. In an impure sample, this percentage would be different.

Your teacher suggests the following method for determining if the sample is pure or not:

1. Measure the mass of your empty test tube using a mass balance.
2. Measure out a sample of the Mad Scientists Powder and determine its mass using a mass balance
3. Heat the powder in a test tube over a Bunsen burner until all of the water has evaporated.
4. Measure the mass of the now dehydrated powder.
5. Use the following formula to calculate the percentage of the original powder that was water:
$\text{% water} = \frac{\text{Mass of water removed}}{\text{Original mass of powder}} \times 100$

[To determine the mass of water removed, subtract the mass of the heater powder from the original mass of the powder.]

Once you have determined the percentage of water in the original sample, you must decide whether the sample is pure or not. Your teacher suggests that if the percentage of water is between 33-39%, then we can consider the sample as pure.

The prac exam will consist of 2 stages:

Stage 1:     Data collection

In stage 1, you will be given access to a science lab and all of the equipment needed to conduct your experiment. You must collect the data required in this time. Make sure that you follow all safety procedures,  and that you leave your work space neat and tidy.