The Solubility Curve of KNO3
Theory
A solution is composed of a substance called the solute dissolved in another substance called the solvent. The solution forms because the attractive forces between the solute and solvent are similar. Thus, a polar solute dissolves a polar solvent while non-polar solute requires a non-polar solvent. A substance that is a solid, liquid, or gas may be the solute or solvent. However, most of the solutions we work with contain water as the solvent (water is also known as the universal solvent).
Some solutions form quickly and others will form slowly. The rate at which a solution is made is depended on the size of the solute particles, the amount of agitation to the mixture, and/or the temperature of the solvent. This lab will focus on the amount of solute that can be dissolved in a solvent at different temperatures; the amount of solvent will remain constant.
Purpose
To graph the solubility curve of KNO3 using the same volume of water at different temperatures.
Safety
Safety glasses and closed-toed shoes must be worn at all times in the laboratory.
Procedure
1. Create a hot water bath using a 250 mL beaker, ring stand, O-ring, wire gauze, and a Bunsen burner.
2. Create an cold water bath using a 250 mL beaker, ice, and water. Fill the beaker 3/4 with the ice and water.
3. Obtain a piece of weighing paper. [use the same one for entire experiment]
4. Obtain a large test tube and a test tube holder.
5. Place exactly 10.0 mL of water in a large test tube
6. Place between 2.50 –3.50 grams of KNO3 onto the paper. Record the mass. Add the KNO3 to the 10.0mL of water in the test tube.
7. Using your test tube holder, place the test tube in the hot water bath stirring until it is all dissolved. With the test tube in the hot water bath, stir heat gently until all of the KNO3 dissolve.
8. Then, remove the test tube and SLOWLY cool the solution by repetitively placing the test tube in your cold water bath for a short period and then removing it. Be certain to constantly sir. Record the temperature at which the first crystals of solid KNO3 appear.
9. Repeat steps 6 – 8 four more times to same 10.0mL of water (thus, the concentration of the solution will continue to increase each time). Be certain to always record the exact amount of KNO3 and the temperature at which the first crystals appear (you should have five masses and five temperatures total).
**Pre-Lab: Go to WebAssign and complete assignment
Table/Observations
Make an appropriate table based on the procedures. A sample is given below:
Mass of KNO3Weighed each trial / Temperature when crystals form / Total mass KNO3
in the test tube / Solubility in
g of solute/100g H2O
Calculations
1. Calculate the mass of KNO3 used in solution for each trial (column #3). Record your answers for each trial in your data table.
2. Calculate the solubility of each solution (the maximum grams of solute in 100 grams of water). Since we used 10.0 mL of water and the density of water is 1.0 g/mL, the mass of water is 10.0 g. Thus, multiply the grams of solute and the grams of water by 10 to obtain the solubility of each solution. Record these values into your data table (column #4).
2. Using the data from your table, create a solubility curve for KNO3. Plot the solubility in g KNO3/ 100 g H20 on the y axis and the temperature on the x axis.
Questions
1. According to your graph, how does the solubility of KNO3 change as the temperature increases?
2. On your graph, what is the change in solubility from 30ºC to 60ºC?
3. At what temperature does KNO3 have a solubility of 50 g/ 100 g H20?
4. The solubility of sucrose (sugar) at 70ºC is 320g/100 g H20.
- How much sucrose can dissolve in 200 g of water at 70*C?
- Will 400 g of sucrose dissolve in a container that holds 200 g of water?
5. If the solubility of sucrose at 0ºC is 180 g/100 g H20, will 400 g of sucrose dissolve in a pitcher of 200 g of iced tea at 0ºC? If not, how many grams will?
References:
Phillips, J., et al. Chemistry: Concepts and Applications. New York: Glencoe
McGraw-Hill, 1997.