Solubility of a Salt

Introduction

Solubility is defined to be the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature. Typically, solubilities are recorded as grams of solute per 100 g of solvent. For many solutes, increasing the temperature increases the solubility, that is, more of the solute dissolves in the same amount of solvent. In this experiment, you will characterize the temperature behavior of a solution made up of KNO3 dissolved in water.

As noted below, you will begin by combining different amounts of KNO3 in similar amounts of water. Most of these systems will require heating before the solute will completely dissolve. After the solid completely dissolves, the solution is constantly stirred and allowed to cool. At some point, the temperature will be low enough for the solution to become saturated. This point is indicated by the formation of small crystals (that look like snowflakes) within the solution system.

Noting the temperature when crystals form and knowing the amounts of solute and solvent permit us to construct a solubility graph. This is a graph of solubility (y axis) vs. temperature (x axis). After plotting the temperature and solubility for all of your solutions, you will connect the dots with a smooth line used to interpolate solubilities between the dots.

Lastly, you will be given an unknown solution. After experimentally measuring the temperature corresponding to the appearance of crystals, you will use your previously constructed solubility graph to determine the unknown's concentration.

Apparatus:

http://webs.anokaramsey.edu/chemistry/Chem1062/Images/img.gif

Procedure

1.      Construct the apparatus shown above. Note that initially, heating and stirring are accomplished by using the hot/stir plate. As the solution is cooled, a second cool hotplate (or stir plate) is used to stir the mixture.

IMPORTANT: Safety goggles must be worn for this experiment!
  

2.      Obtain six vials from the lab instructor and label appropriately. Mass each vial (cap on) and record each respective mass. On a piece of weighing paper, measure out the amount of KNO3 given by the table below. Transfer the solid to its vial and re-mass to determine the amount of solid actually transferred to the vial. Repeat this procedure for all six vials.

Vial #

Approximate KNO3 mass (g)

1

~3.0 g

2

~2.5 g

3

~2.0 g

4

~1.5 g

5

~1.0 g

6

~0.5 g
  1. Into each vial, pipette 2.00 mL of deionized water. Re-mass each vial to determine the amount of water actually transferred to the vial.
     
  2. Connect a stainless steel temperature probe to the LabPro interface and open the Vernier LoggerPro application. From the "Insert" menu, select a digital meter. There is no need to calibrate the temperature probe, if the stainless steel probe is used.
     
    IMPORTANT: Position all wires away from the hot plate!!! Always be watchful of the wires whenever you reposition the apparatus!
     
  3. Remove the cap from vial #1 and insert a small magnetic stir bar. Use clamps to support the vial in the hot water bath (a 50-mL or 100-mL beaker). Note that the bottom of the vial must be close to the bottom of the water bath for the magnetic stirrer to work.
     
  4. Continue heating the solution until all of the solid has dissolved. Do not leave the vial in the water bath any longer than what is required to completely dissolve the solid.
     
  5. When the KNO3 has dissolved, move the apparatus to a cold stir plate and continue stirring. Insert the temperature probe into the vial, making sure that the probe is clean and dry. Allow the vial to cool slowly (it will be necessary to construct a cold water bath in a small beaker to accelerate the process in some cases).
     
  6. At some point, crystals will form and gradually build in intensity as the solution cools (snow storm). Record the temperature on the digital meter at the instant crystals form. If the appearance of crystals is very sudden, supercooling of the solution may have occurred and it will be necessary to repeat the trial reheat the apparatus and repeat the cooling process (always stirring) as necessary to obtain a good, reproducible temperature measurement. You may need to use cool water or an ice bath to obtain low enough temperatures to attain crystal formation. Don't cool too quickly or you may miss the appearance of crystals.
     
  7. Empty the vial in the appropriate waste container. Repeat steps 5-8 with each of the numbered vials.
     
  8. Obtain an unknown KNO3 solution from the laboratory instructor (record the unknown number and include this in your report). Heat the solution until the solid completely dissolves. Cool the solution and record the temperature when crystals form. Repeat as necessary to obtain a reproducible temperature measurement.

Data Analysis and Questions

  1. Use Microsoft Excel to construct a data table containing columns labeled with Vial number, T (ºC), g KNO3, g H2O, and Solubility (gsalt/100 gsolvent) for all six trials. Include a separate entry for the unknown solution's identification number, its characteristic temperature, and its concentration.
     
  2. Construct a solubility graph using the temperature and solubility information for the first six trials. (Use the scatter plot option, but do not connect the dots.)
     
  3. Perform a linear trendline analysis on the data. Then try a second order polynomial trendline analysis. Note and record the equation and correlation coefficient (R2) for each trendline. The equation that best fits the data will have a correlation coefficient closest to 1.
     
  4. Calculate the concentration of your unknown solution using the equation for the trendline that best fits your data. Confirm this result visually by checking the position of the data point on your trendline.
     
  5. Answer the following questions:

A.

Using your graph, tell if each of these solutions would be unsaturated, saturated, or supersaturated:

i.

110 g of KNO3 in 100 g of H2O at 40°C

ii.

60 g of KNO3 in 100 g of H2O at 70°C

iii.

140 g of KNO3 in 200 g of H2O at 60°C

B.

According to your graph, will 50 g of KNO3 completely dissolve in 100 g of H2O at 50°C? Explain.

C.

Using your trendline equation, how many grams of KNO3 will dissolve in 100 g of H2O at 30°C?

Lab Report
You may choose to submit an individual or group report for this lab. Follow the guidelines for Laboratory Reports at http://webs.anokaramsey.edu/chemistry/Chem1062.  Please read these guidelines very carefully before beginning the report, especially if you did not take Chemistry 1061 lab last semester.  In your report, please include a title, procedure, results, and discussion.  In the discussion, please make sure to include the identity of your unknown and how you were able to make this determination.  The answers to question 5 should be included as an appendix at the end of the report.

Follow your instructor’s directions for submitting this lab report. Remember to name the file as specified (Lastname_Solubility or Lastname1_LastName2_Solubility). If you are emailing your report, use the subject line “Chem 1062: Solubility Lab”. If you worked in pairs and are submitting this assignment on an individual basis, please underline your own name and include your lab partner’s name on the assignment.

Lab courtesy of Vernier Software, "Chemistry With Computers"; and Kirk Boraas, Minneapolis Community and Technical College. Edited by the Anoka-Ramsey Community College Chemistry Dept. Last update was April, 2011.