Lab #8  Solubility Curves 
Purpose:  To show how the solubility of a salt such as ammonium chloride varies with the temperature and how to plot its solubility curve on the basis of observed data.
 
Apparatus:  Balance, sensitive to 0.01 g;  400 mL beaker;  Bunsen burner;  10 mL pipet;  iron ring; test tubes; thermometer and wire guaze
 
Introduction:  The solubility of a solute is the amount of solute dissolved in a given amount of a certain solvent at specified temperatures.  The usual units used on solubility graphs are grams of solute per 100 mL of water at specified Celsius temperatures. Increasing the temperature usually increases the solubility of solids in liquids. Decreasing the temperature usually decreases the solubility.  In this experiment we will determine the solubility of ammonium chloride at various temperatures by allowing a hot solution of the compound to cool and observing the temperature at which the solid begins to crystallize.
 
Procedure:
1. Weigh  4.00 g, 5.00 g, 6.00 g and 7.00 g samples of ammonium chloride.  Put each sample in a properly labelled test tube.

2.  Using a pipette, measure 10.0 mL of water into each test tube.

3. Assemble your retort stand with a ring and wire guaze.  Fill the beaker half full of water, put the 4.00 and 5.00 gram samples in the water, and bring the water to a boil.

4.  While this is going on, heat the test tube containing the 7.00 gram sample with the bunsen burner.   Stir continuously until all the solute dissolves.  DO NOT bring the contents of the test tube  to a boil.

5.  With the thermometer in the solution, allow the contents to cool while stirring gently.

6. Record the temperature at which the solid begins to crystallize (when the first crystals appear.)

7.  Heat the solution in the test tube again to dissolve the crystals, allow it to cool again, and recheck the temperature at which crystallization occurs.

8.  Repeat this procedure with the test tube containing the 6.00 gram sample.

9.   Repeat with the 5.00 and 4.00 gram samples using the hot water bath as a source of heat, rather than the burner.

10. A cold water bath is needed to hasten the crystallization of the 4.00 grams sample.
 

Data Table
                       Solubility (g/10.0 mL H2O)           Temperature 
                   oC
  4.00 g  ..........................................................................    
  5.00 g  ..........................................................................     
  6.00 g  ..........................................................................     
  7.00 g ..........................................................................     

 
Calculations:
1.  Convert your grams/10 mL values to grams/100 mL values.

2.  Plot the solubility curve for ammonium chloride from the results in question 1.  Along the horizontal axis on the graph paper,  plot the temperature scale using 10 degree intervals.   Place the points representing the four masses along the vertical axis.  Label it as grams of solute/100 mL water.

3.  Draw a small circle around each dot.  This represents the degree of error and construct a smooth curve for the observed data.  (This is not kindergarten, DO NOT play connect the dots.)

4. Extrapolate this curve to the temperature limits of your graph by using a dashed line to indicate extrapolation.

5.  Here is the official solubility data for NH4Cl in grams/100 mL H2O
    30oC   41.4 g
    50oC   50.4 g
    70oC   60.2 g
    90oC   71.3 g

Plot these points on your graph using a different colour.  Identify the lines as "experimental" and "official."
 

Questions:
1.   From your solubility curve determine the solubility of ammonium chloride in g/100 mL of H2O at 60oC.

2.  Consult a Handbook of Chemistry and record the accepted value for the solubility of ammonium chloride at 60oC.

3.  Calculate your percentage error from the answer to questions 1 and 2.