Activity 6: Determination of an Unknown

Students will work in groups of 2-4.

1. Acquire an unknown substance from the teacher and record the number.

2. Determine the mass of the unknown.

3. Acquire and weigh a 500mL beaker.

4. Add 300mL of water to the beaker and re-weigh.

5. Take the temperature of the water and add some solute slowly to the water.

6. Stir until it dissolves. Keep adding more solute a little bit at a time until it doesn't dissolve anymore.

7. Record the temperature again and weigh the mass of solute remaining (calculate how much was added to the water).

8. Place the beaker on a hot plate set to low, and allow the temperature to rise and then stabilize.

9. Repeat steps 5-7.

10. Increase the temperature of the hot plate, and repeat steps 5-7 (should have recordings for temperatures of 20, 40, 60, 80, and 100 degrees Celsius).

11. Plot your data and draw your solubility graph (with labels).

12. Compare your graph against those that are provided and determine what your substance was.

13. Allow the solution to cool rapidly.

14. Answer the following discussion questions.

Discussion Questions

1. Why doesn't the solute dissolve anymore (as occurred in step 6)?

2. What was the number of your unknown?

3. What substance was it? Explain.

4. What happens when the solution is allowed to cool? Why does this occur?