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DERIVING A SOLUBILITY CURVE

Aim: To plot the solubility curve of potassium nitrate on the basis of experimental data.

Materials: Refer to sheet

Method: Refer to sheet

Discussion:

0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100

120

140

Solubility of KNO₃ in Water

Temperature (⁰C)

g/10

0g o

f wat

er

1A. The solubility of potassium nitrate at 50⁰C is 52g/100g of water.

1B. The temperature at which the solubility of potassium nitrate is 80g/100g of water is 60⁰C

1C. The maximum amount of potassium nitrate that would dissolve in 100g of water at 60⁰C is 80g

1D. The temperature to which 20g of potassium nitrate in 25g of water needs to be heated in order to completely dissolve is 60⁰C.

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1E. The mass of potassium nitrate that would crystallise out of a saturated solution containing 100g at 90⁰C if the solution were cooled down to 30⁰C would be 164g

2. Reasons for possible errors in the above calculations: It is difficult to identify the exact crystallisation point Measurement of water or potassium nitrate is inaccurate Recordings of temperature may not be accurate

3.

Conclusion: By following the method above, it is easy to find out the solubility of potassium nitrate as well as many other substances in water. The observations found during the experiment were used to construct a graph which could be used to determine the solubility of that substance at any particular temperature or weight of solute.

1. Weigh 6.5g of potassium nitrate in a

large test tube and add 5ml of water

2. Heat the test tube by placing it into boiling water

and stir contents with a thermometer until all the crystals have dissolved.

3. Remove test tube from boiling water and continue stirring until first signs of

crystallisation are detected. Record the temperature.

4. Add 3ml of water into test tube and repeat steps 2 & 3

5. Repeat step 4 until crystallisation fails to occur even when test tube is cooled in ice

6. Use results and formula [6.5/(amount of water) x 100] to find amount of

potassium nitrate in 100g of water

7. Construct a table which shows the masses of water

added to potassium nitrate, the solubilty of potassium nitrate and temperature recorded.

8. Draw a solubility graph with temperature on the

horizontal axis and solubility on the vertical axis.