Solubility Curve

Download solubilitycurve.ddw

Contributed by :

Sandra Curry
Western Heights College
Geelong
Victoria
Australia

Purpose

To plot the solubility curve of an ionic compound, potassium nitrate, on the basis of experimental data.

Duration

50-80 minutes

Materials

• 6.5g potassium nitrate
• large test-tube
• burette
• retort stand, boss head and clamp
• 600mL beaker
• electronic balance
• electric urn, filled with boiling water
• temperature sensor, 0 to 110(C, and data collection system
• spatula
• ice
• safety glasses
• computer and spreadsheet program
  

Safety

Wear safety glasses and a laboratory coat for this experiment.

Procedure

1. Turn on the temperature SensorMeter and make sure the 0 to 110°C range is highlighted. (If not, use the up and down arrows to highlight the correct range.)

2. Log onto the computer and launch Datadisc32.

3. From the menu, select Set Up - Calibrate. Then work through the screens:
Sensormeter Range for Channel 1 : (Temperature) 0 to 110°C
Channel to calibrate : Channel 5 (available for keyboard entry)
Calibration type : enter calibration
Sensor values : Channel name : Solubility (g) KNO3 (cannot subscript 3)
Channel unit : 100g water (first 1 appears superscripted - "glitch")
Minimum value : 20
Maximum value : 150
(Leave resolution and error at zero.)
Calibrate another channel? : No

4. Go back to the menu, and choose Measure - Record. The graph will now appear on the screen - Maximise. At Click here to set title, type in "Deriving a Solubility Curve" and Enter. At X Channel, click the down arrow and select 1 Temperature for the x-axis.

5. Next, weigh accurately, in a large test-tube, 6.5g of potassium nitrate. This is most easily done by standing the test tube upright in a beaker.

6. Use a burette to add exactly 5mL of distilled water to the test-tube.

7. The solubility of 6.5g of potassium nitrate dissolved in 5.0g of water is equivalent to a solubility of 6.5/5.0 x 100, i.e. the solubility of 130g of potassium nitrate in 100g of water. Subsequently, 3.0mL increments of distilled water will be added. A table which shows the mass of water added to the potassium nitrate and the corresponding solubility of the potassium nitrate is shown below.

8. Go to the graph and click on the Start recording icon (looks like an interface with ports, or a pair of glasses). At the X+ (Set X value) icon, type 130.0 in the right hand column (the first column is the last temperature reading) and Enter. The cursor will move to this point on the graph.

9. Warm the beaker by filling with boiling water, then emptying. Refill with boiling water.

10. Add the test-tube, insert the temperature sensor and stir the contents carefully with the sensor until all the crystals dissolve.

11. Remove the test-tube from the boiling water and continue to stir gently until the first signs of crystallisation are detected. Immediately click on the Record icon (same one as before) on the screen.

If, at any time, you take a reading you are not happy with, re-do it. You can edit the data later.

12. Add a further 3.0mL of water from the burette into the test-tube. Change the X (channel 5) value to 81.2 and Enter.

13. Place the test-tube back in hot water until all of the crystals have redissolved. Remove and again watch for the first signs of crystallisation - Record.

14. Continue adding 3.0mL increments of water, changing the X (channel 5) value and recording the temperature at crystallisation as above. To speed up this process, the test-tube may be cooled by placing it in a beaker of cold water, as long as the solution is stirred so that the temperature throughout the solution is kept uniform. Eventually, a stage will be reached where crystallisation on cooling fails to occur even when the tube is placed in ice.

15. At this point, click on the Finish recording icon (a red stop hand). Choose Keep this data. The completed graph will appear on the screen (Maximise if necessary). Go to Graph - Plot options - Include zero on axes and Show full graph grid (first channel) - OK.

16. To see the table of temperature values, click on the Table icon. Delete any incomplete data or wrong readings by highlighting the row, then Edit - Delete - Delete data row(s)? - OK.

17. From the menu, select Data - Data information and enter your names for identification.

18. To insert a trendline, go to Data - Fit function - Y=AeBx - Set range : x range 0 to 130.0 - Calculate - Close. Go to Graph - Plot all data. If you are not happy with this line, go to Data - Delete channel - Channel 2 - OK, (then Graph icon again), and draw in a line-of-best-fit later.

19. Save the file. Choose File - Save, then:
File name : Solubility Curve
Save file as type : Datadisc Windows files

20. To print the graph and data, go to File - Print - Information, Graph and Table : all data in table - OK. (Unfortunately, each prints on a separate page. To combine, you would need to copy and paste to a word processing document.)

Theory

The solubility of a solute measures the amount of solute that can dissolve in a given amount of solvent at a given temperature. In this experiment, you will determine and graph the solubility of potassium nitrate (in grams of potassium nitrate per 100g water) at various temperatures.

In your calculations, you will need to assume that the density of water is equal to 1g mL^-1 at all temperatures. Although the density of water does vary with temperature, this is a reasonable approximation to make in view of the other errors associated with this determination.

Therefore, when 5.0mL of water is added to the solute, it can be said to be equivalent to the addition of 5.0g of water, and so on.

Questions

1. From your graph, determine:

a. the solubility of potassium nitrate at 50°C;

b. the temperature at which the solubility of potassium nitrate is 80g/100g water;
c. the maximum amount of potassium nitrate that would dissolve in 100g of water
at 60°C;

d. the temperature to which 20g of potassium nitrate in 25g of water needs to be
heated in order to completely dissolve.

2. Suggest reasons for possible errors in the above determinations.