Lab Activity 16: Solubility of a Salt

Pre-Lab Discussion:
The solubility of a pure substance in a particular solvent is the quantity of that substance that will dissolve in a given amount of the solvent. Solubility varies with the temperature of the solvent. Thus, solubility must be expressed as quantity of solute per quantity of solvent at a specific temperature. For most ionic solids, especially salts, in water, solubility varies directly with temperature. That is, the higher the temperature of the solvent (water), the more solute (salt) that will dissolve in it.
In this experiment, you will study the solubility of potassium nitrate (KNO3) in water. You will dissolve different quantities of this salt in a given amount of water at a temperature close to the water's boiling point. Each solution will be observed as it cools, and the temperature at which crystallization of the salt occurs will be noted and recorded. The start of crystallization indicates that the solution has become saturated. At this temperature, the solution contains the maximum quantity of solute that can be dissolved in that amount of solvent.
After solubilitv data for several different quantities of solute have been collected, the data will he plotted on a graph. A solubility curve for KNO3 will be constructed by connecting the plotted points.

Purpose:
Collect the experimental data necessary to construct a solubility curve for potassium nitrate (KNO3) in water.

Safety:
Tie back long hair and secure loose clothing when working with an open flame. Be sure you use a test tube holder when removing tubes from the hot water bath. Always wear safety goggles and a lab apron or coat when working in the lab.

Procedures:

While one lab partner carries out the instructions in steps 1 through 4, the other partner should go on to step 5.
1. Using a marking pencil, number four test tubes 1 through 4. Place the tubes in a test tube rack.
2. On the balance, measure out exactly 2.0 g of potassium nitrate (KNO3). Pour the salt into test tube #1.
3. Repeat step 2 for the following masses of KNO3. Add each quantity to the test tube indicated: 4.0 g to test tube #2, 6.0 g to test tube #3, 8.0 g to test tube #4
4. Add exactly 5.0 mL of distilled water to each test tube.
5. Fill a 400-mL beaker about three-fourths full of tap water. This will be used as a water bath. Using the water bath and test tube #1, prepare the setup shown in Figure 1. Heat the water to 90°C and adjust the flame to maintain the water at about this temperature.
6. Stir the KNO3-water mixture with a glass stirring rod until the KNO3 is completely dissolved. Remove the stirrer and rinse it off. Loosen the clamp and, using a test tube holder, remove the tube.
7. While lab partner number one repeats step 6 for test tube #2, lab partner number two should place a warm thermometer (dipped into the hot-water bath) into the solution in test tube #1. Hold the test tube up to the light and watch for the first sign of crystallization in the solution. At the instant crystallization starts, observe and record the temperature. Should crystallization start too quickly (because of a cold thermometer), redissolve the solid in the hot-water bath and repeat this step.
8. Steps 6 and 7 should be followed for all four test tubes. One lab partner should stir the KNO3 until it dissolves, and the other partner should record the temperatures of crystallization. Record all temperatures in your data table (see data below).
9. If any doubtful results are obtained, the procedure can be repeated by redissolving the salt in the hot-water bath and allowing it to recrystallize.

Data:

• Create a data table to indicate the temperature of crystallization for each test tube (include the "grams of KNO3 per 5.0 mL H20"). Also, in this table, include a column called "Grams of KNO3 per 100 mL H20" and convert your data appropriately.
• Create a graph and plot your experimental data on the graph. X axis = Temperature, Y axis = Solubility of KNO3/100mL H20.

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