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HandoutThe Solubility of a Salt |
In this experiment, you attempted to determine the solubility of a salt (KNO3) at various temperatures, and plotted a solubility curve for the salt. The solubility of the salt refers to the mass of the salt which will dissolve per 100 mL of solvent (in this case, water) at a particular temperature. To do this, you took a fixed amount of salt, and determined at what temperature the solution became saturated for a given amount of solvent.
The basic data you record consists of (1) the mass of salt taken (which should have been approximately 5 grams); (2) the volume of water in which the salt was being dissolved (measured with a buret); and (3) the temperature at which crystals first appeared in the solution as the solution was cooled from 100◦C. The mass should have been recorded at least to the third decimal place (for 4 significant figures overall); the volumes should have been recorded to the nearest 0.01 mL (for either 3 or 4 significant figures); the temperature should have been recorded to the nearest 0.1◦C. The following table reproduces only a few lines of your data table (since the calculations are the same for each volume of water used). We assume that the density of water is always 1.000 g/mL.
| mass of salt taken = 5.021 g | ||
| mL water used | Saturation Temperature | solubility (g solute/100 g H2O) |
| 3.00 mL *see below | 87.2◦C | 167 g solute/100 g water at 87.2◦C |
| 4.01 mL | 79.8◦C | 125 g solute/100 g water at 79.8◦C |
| 5.15 mL | 68.9◦C | 97.5 g solute/100 g water at 68.9◦C |
| 6.01 mL | 60.2◦C | 83.5 g solute/100 g water at 60.2◦C |
For the sample in which 3.00 mL of water had been added to the salt, the solubility would be calculated as
(5.021 g solute/3.00 g water) × (100 g water) = 167 g solute
After you calculate the solubility of your salt at each temperature you determined, you will make a graph of your data which represents the solubility curve for your salt. Solubility curves are typically not straight line graphs, so you will have to use a "French curve" to draw a smooth curve between your data points (available in the bookstore if you don't have one). The solubility curve for potassium nitrate, the salt you used, is given as an example in most text books because the solubility of KNO3 varies more with temperature than does the solubility of most salts. Solubility curves for several salts are shown below: Make sure your graph is made following the graphing guidelines given in your lab manual.
Questions
The questions come in two sets: a set of "Pre-Laboratory" Questions and a set of Questions to answer after you perform the experiment. You should submit both with your report.
Pre-Laboratory Questions
1. These two definitions should be easy to find in your textbook.
2. Think of what the term "solubility" means (see answer in #1 above). Solubility is an equilibrium process. The position of an equilibrium is not affected by things such as "stirring the solute".
3. There are many examples of "dynamic equilibrium" in your textbook. Try to find a general definition of it. After you define it, look back at your answer to #2 and see if it makes sense.
Questions
1. Why could this be? What about the heating process would make it undesirable to use as opposed to letting the test tube cool spontaneously? When you heated your beaker...do you suppose the temperature was completely uniform throughout the beaker? Or were their "hot spots" because part of the beaker was in contact with the flame?
2. This one is so simple it's easy to miss: The solution was made to be almost saturated so that it would become saturated when you started to cool it, and the crystals would appear quickly. Virtually any amount of water could have been used for the first run, but by using an almost saturated solution, we were able to do multiple runs using the same sample of salt and different amounts of water added.
3. Why would you want to minimize the time the solution in the test tube was kept at the boiling water temperature? Although a stopper was used, remember that the stopper had an opening in it. What could happen to the water in the solution because of the opening in the stopper?
