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g of KNO3 temp craystized (C)
1.533
22.0
3.553
44.9
5.590
66.9
7.640
76.8
M (mol/L) Temp (K)
3.03E-04
295.0
7.03E-04
317.9
1.11E-03
339.9
1.51E-03
349.8
(-RT ln ksp
180
160
Solubility (g/100mL)
delta g
Solubility (g/100mL)
Temp (C)
30.66
22.0
71.06
44.9
111.8
66.9
152.8
76.8
140
120
100
80
60
40
20
0
1
check
delta G
M^2
M^2
LN M^2
RT
delta G
delta G (J/mol)
4.01E+04 9.21E-08
9.12 2.21047 -2476.23 -5.47E+03 3.98*10^4
3.87E+04 4.95E-07
4.94 1.597365 -2668.45 -4.26E+03 3.87*10^4
3.88E+04 1.22E-06
1.22 0.198851 -2853.12 -5.67E+02 3.85*10^4
3.81E+04 2.29E-06
2.29 0.828552 -2936.22 -2.43E+03 3.78*10^4
Temp (K) delta G
295 -5473.632
317.9 -4262.494
339.9 -567.3455
349.8 -2432.811
0
Solubility Table of KNO3
Delta G (J/mol)
-1000
290
300
310
320
330
-2000
-3000
-4000
-4262.493671
-5000
-5473.631643
-6000
1
2
3
Temp (C)
4
Temp (K)
330
340 -567.3454814
350
360
-2432.811412
4262.493671
Temp (K)
g of KNO3 temp craystized (C)
1.533
22.0
3.553
44.9
5.590
66.9
7.640
76.8
Solubility (g/100mL)
Temp (C)
30.66
22.0
71.06
44.9
111.8
66.9
152.8
76.8
M (mol/L)
3.03E-04
7.03E-04
1.11E-03
1.51E-03
Temp (K)
295.0
317.9
339.9
349.8
Temp (K) delta G
295
-5450
317.9
-10300
339.9
-13600
349.8
-15800
Solubility and Thermodynamics
In this lab you will construct a solubility curve for the salt KNO3. Then, from that solubility curve, you will
determine the thermodynamic values of G, H, and S for the process of dissolving KNO3.
Part 1 – Solubility Curve for KNO3 (to be completed during the first lab period)
1. You will prepare a solubility curve for KNO3 over the temperature range of about from room
temperature up around 75oC. To start this process, you will first determine the solubility of KNO3 at
room temperature. Do this in a test tube containing 5.00 mL of distilled water. Discuss as a group how
you will measure this. Write down a step-wise procedure. Before you perform your measurement,
check with your instructor. Once you have the instructor’s okay, carry out the measurement and record
your result on your data sheet. Retain the mixture you made for future use.
2. Next, you will determine if the solubility of KNO3 increases or decreases with temperature using your
solution from step 1 above. Make a hot bath (beaker of water on a hot-plate) and an ice-water bath.
You will place the test tube in each of these solutions. Before you do this, answer the following
questions and show your instructor.
a. What type of solution do you have in your test tube right now: unsaturated, saturated, or
supersaturated?
b. If the solubility of KNO3 increases with increased temperature what will happen when you place
the test tube in the hot bath? The cold bath?
c. If the solubility of KNO3 decreases with increased temperature what will happen when you place
the test tube in the hot bath? The cold bath?
*Retain your test-tube of KNO3 for the next step. If your solution is warm, let it cool to room temp.
before proceeding with the next step (you can put it in the ice-bath to speed the cooling process).
3. Next, you will measure the solubility of KNO3 at a higher temperature. You will do this a little bit
differently than how you did it at room temperature. Add a little more KNO3 to the test tube and record
the exact amount added. Heat the test tube in the hot bath until all of the solid dissolves. Place a
temperature probe in the test tube. Remove the test tube from hot bath to let it cool. Observe the
solution carefully and watch for precipitate/crystals to just begin to form. Record the temperature at
which this occurs.
a. What type of solution do you have at the point crystals begin to form: unsaturated, saturated,
or supersaturated?
b. Assuming the mass of the crystals is negligible, can you calculate the concentration of the
solution is at this point? What is it?
c. If you know the answers to the above two questions, then you know the temperature at which
a solution of this concentration becomes saturated. In other words, you know the solubility
limit at that temperature!
At this point you should have two temperature/solubility data points, but you will need more to construct
a nice solubility curve for KNO3. Repeat step 3 with different amounts of KNO3 to find the solubility at
different temperatures. Keep track of your data in table form on your data sheet. When you think you
have enough data for a nice curve, make a graph on Excel and show your instructor. *Be sure to save a
copy of your Excel file or email it to yourself. You will need this data for part 2 of this experiment.
4. Use your graph to estimate the solubility of KNO3 at 10oC, 50oC, and 90oC. Show instructor.
**Completing 1-4 above will constitute your “group planning questions” for this experiment.
**Be sure to completely record all your data/observations on the provided data sheets (grid paper). This will be
collected as your “individual notes” for this experiment.
Part 2 – Thermodynamics of the KNO3 dissolution process
Dissolving KNO3 in water can be viewed as an equilibrium process. As solid KNO3 dissolves, it dissociates
into its component ions. When a solution is saturated (is at the solubility limit), some KNO3(s) is not
completely dissolved and we have both the solid and the ions present in equilibrium with each other.
KNO3(s) ⇌K+(aq) + NO3-(aq)
1) Write the equilibrium expression for this reaction.
2) Calculate the value of the equilibrium constant for this reaction at each of the temperatures found in Part
1 of this experiment. (Hint: You should be able to calculate concentration of the ions at this point using
the data from your solubility graph. To simplify things, you may approximate the solution volume to be
the same as the solvent volume.)
3) Calculate G ° at each temperature using the equilibrium constants you found in #2 above.
4) Use Excel to make a plot of G versus the temperature in Kelvin. Fit a linear trendline to this data and
display the trendline equation and R2 value on the plot.
5) We know that the free energy change for a reaction can be expressed as G = H − TS. This
expression can easily be rearranged to
G = −TS + H
This is a linear equation (y = mx + b). You fit a linear equation to your data in step 4 above. What do
the slope and intercept of your graph correspond to?
6) Calculate the values of H and S for the reaction KNO3(s) ⇌ K+(aq) + NO3-(aq).
**Show the instructor your answers for 1-6 above. This will constitute your group summary questions for
this experiment**
Lab Report
Following the lab report guidelines already provided, you will write a lab report that summarizes the results
from both part 1 and part 2 of this experiment. (E.g., provide a solubility curve and all of the important
thermodynamic values calculated in part 2). Your introduction should provide the background on how the
solubility was measured and how the thermodynamic values were calculated.

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