Nick DiPreta
Jonny Masci
Solubility of a Salt
Due: 3/22/10
Pre Lab- The solubility of a pure substance in a solvent is the amount of that substance that will dissolve
in a given amount of the solvent. Solubility changes according to the temperature of the solvent.
Solubility is the quantity of solute per quantity of solvent t a specific temperature. The higher the
temperature of the solvent, then the more solute that will dissolve in it. In this experiment, we studied
the solubility of potassium nitrate in an amount of water at a temperature that is close to water’s boiling
point. Each solution will be watched as it cools until tehe temperature at which crystallization of the salt
occurs and can be recorded. The start of crystallization demonstrates that the solution is saturated.
Around this temperature, the solution has the most amount of solute that can be dissolved in the specific
amount of solvent. The solubility data for the different quantities of the solute will be plotted on a graph.
A solubility curve for KNO3 will be made by plotting the points.
Purpose:
To collect the experimental data needed to make a solubility curve for KNO3 in water.
Equipment
Balance,burner, microspatula, test tubes, test tube holder, test tube rack, beaker, thermometer, graduated
cylinder, stirring rod, ring stand, iron ring, wire gauze, marking pencil, safety goggles, lab apron
or coat
Materials
Potassium nitrate, distilled water
Safety
Tie back all long hair and secure clothing when working with the glame. Use a test tube holder when
removing the tubes from the water bath. Note all caution alert symbols.
Procedure
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 two grams of potassium nitrate. Pour the salt into test tube 1.
3. repeat step 2 for the following masses of KNO3. Add each amount 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 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 prepare the setup shown in figure 30-1. Heat the water to 90 degrees
Celsius amd adjust the flame to maintain the water at 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. Have the other lab partner place the warm
thermometer dipped into the warm solution into 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 occurs
observe and report the temperature. Should crystallization start to quickly, re-dissolve the liquid
and restart this part.
8. Steps 6 and 7 should be followed for all four test tubes. One lab partner should stir the potassium
nitrate until it dissolves, and the other should record the temperature.
9. If any inaccurate results are obtained, repeat the procedure by reheating in a hot water bath.
Observations and Data
Test tube #
1
2
3
4
Calculation
Grams of KNO3/5.0mL H20
2.0g/5.0mL
4.0 g/5.0 mL
6.0g/5.0mL
8.0g/5.0mL
Crystallization temperature
(Celsius)
32 degrees
56 degrees
69 degrees
84 degrees
1. Using Proportions convert the experimental mass/volume ratios to equivalent mass/100mL ratios.
2.0g/5.0 mL= 40g/100mL
4.0g/5.0mL= 80g/100mL
6.0g/5.0mL= 120g/100mL
8.0g/5.0mL= 160g/100mL
2. Plot your experimental data on the grid provided. Plot mass of solute per 100mL of water on the Y
axis and temperature on the X-axis.
3. Construct a solubility curve by connecting the plotted points on your graph.
Temperature (degrees celsius)
Mass of solute dissolved per 100 mL of water
(grams)
32 56
69
84
40 80 120 160
Mass of solute KNO3 dissolved per 100 mL of
water (grams)
180
160
Mass Of
Solute KNO3
dissolved
per 100 mL
H2O(grams)
Mass of solute dissolved per
100 mL of water (grams)
140
120
100
80
60
40
20
0
0
20
40
60
80
100
Temperature (Degrees Celsius)
Conclusions and Questions
1. How many grams of KNO3 can be dissolved in 100mL of HOH at the following temperatures?
a. 30 degrees Celsius
b. 60 degrees Celsius c. 70 degrees Celsius
A. About 43 grams of KNO3 can be dissolved into 100ml of HOH at 30 degrees Celsius.
B. About 100 grams of KNO3 can be dissolved into 100mL of HOH at 60 degrees Celsius.
C. About 130 grams of KNO3 can be dissolved into 100mL of HOH at 60 degrees Celsius.
2. Define the terms saturated , unsaturated, and supersaturated.
The term saturated defines a solution in which no more solute can be dissolved into at that specific
temperature. The term unsaturated defines a solution in which more solute can be dissolved into
at that specific temperature.
3. Classify the following KNO3 solutions as saturated, unsaturated, or supersaturated. Explain your
answer.
a. 75 grams KNO3/100mL H2O at 40 degrees Celsius.
The solution above is supersaturated, as it is holding about 15 grams extra of potassium nitrate.
b. 60g KNO3/100mL H2O at 50 degrees Celsius.
4. Do the solubillities of all ionic solids increase as the temperature increases? Explain.
Most but not all ionic solid’s solubillities increase as the temperature increases of the solvent. The
addition of more heat energy allows for dissolving by providing energy to break bonds in a solid.
It also makes the molecules “vibrate” faster, allowing for the bonds between the water and the
ionic solid to be formed.
5. How does the solubility of a gas change with increasing temperature? Draw a rough sketch showing
the general shape of a solubility curve of a gas.
Generally, as shown in the graph above, as temperature increases, the solubility of a gas in water
decreases.
6. The graph in Table G of the reference tables shows the solubility curves for several compounds.
Which of the compounds represented on this graph become less soluble as temperature increases?
NH3 and SO2 are two compounds in Table G of the reference tables which decrease in solubility as the
temperature of the solvent increases.
Conclusion- The solubility of a specific salt can be tested in an every day lab, by heating and cooling
specific amounts of the solute in the same amount of water, and observing when the solution becomes
saturated after having 100% of the solute dissolved. We can create solubility curves by using the
experimental data that was obtained, to extrapolate or interpolate different quantities of the solute’s
solubility.