Solubility of a Salt Activity
Names
Period
Solubility is the quantity of a pure substance that will dissolve in a given amount of a particular solvent. Solubility varies
with the temperature of the solvent. Therefore, 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 laboratory activity, 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 its 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 solubility data for several different quantities of solute have been collected, the data will be plotted on a graph. A
solubility curve for KNO3 will be constructed by connecting the plotted points.
Procedure
Every group will be assigned a different mass of KNO3
1. Mass the assigned quantity of KNO3 assigned.
2. Add this quantity to a test tube.
3. Add exactly 5.0 mL water to the test tube.
4. Make a hot water bath by filling a 400 mL beaker about three-fourths full of hot tap water. Heat the water to
~100oC and adjust the dial to maintain the water at about this temperature.
5. Place the test tube into the hot water bath. Stir the KNO3 – water mixture with a thermometer (CAREFULLY)
until the KNO3 is completely dissolved. Remove the test tube from the beaker.
6. 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. If you miss the crystallization point, re-dissolve the
crystals and observe more carefully a second time.
7. Reheat the test tube and obtain a second trial result.
Data Table
Record your results for your classmates to use.
Mass of KNO3/5mL
o
C - Trial 1
o
C - Trial 2
o
C - Trial 3
Avg.
Graph
With this data, make a graph of the mass of KNO3 per 5.0 mL of water on the x axis versus the crystallization
temperature on the y axis. Construct a solubility curve by drawing the best curve.
Questions
1. Identify the solute and solvent in this lab activity.
2. Write an equation for the dissolution (solvation) of KNO3 in water.
3. State the relationship between KNO3 solubility and temperature.
4. Diagram the solvation process for KNO3 in water.
5. Do you think KNO3 is a strong or weak electrolyte? Explain.
6. How many grams of KNO3 can be dissolved in 5.0 mL of water at the following temperatures, using your graph?
a. 30oC
b. 40oC
c. 50oC
d. 60oC
7. Classify the following KNO3 solutions as saturated, unsaturated, or supersaturated. Explain your answer.
a. 4.6 g KNO3/5.0 mL H2O at 25oC (clear solution)
b. 3.1 g KNO3/5.0 mL H2O at 70oC (clear solution)
Group
Mass KNO3
(grams/5mL)
1
3.00
2
3.25
3
3.50
4
3.75
5
4.00
6
4.25
7
4.50
8
4.75
9
5.00
Crystallization
Temperature
#1 (oC)
Crystallization
Temperature
#2 (oC)
Crystallization
Temperature
#3 (oC)
Average