from the absorbance (pathlength = 1 cm)
Calculate the moles of Fe
2+
(moles of Fe
2+
= moles of (NH
4
)
2
Fe(SO
4
)
2
.6H
2
O)
Calculate the molar mass (NH
4
)
2
Fe(SO
4
)
2
.6H
2
O)
Calculate grams of (NH
4
)
2
Fe(SO
4
)
2
.6H
2
O in the crystals
Mass KNO
3
crystals analyzed for purity (from Step B.8.)
Calculate % mass of (NH
4
)
2
Fe(SO
4
)
2
.6H
2
O in the crystals
Crystallization
Number
Summary: % Mass KNO
3
Recovered in Crystallization Steps
Crystallization
Number
Mass KNO
3
Crystallized
% of Original KNO
3
Mass
Grams H
2
O used
Molar Mass KNO
3
Grams KNO
3
Cryst. Temp,
o
C
Solubility
gKNO
3
/100
g water
Molality KNO
3
K
sp
(Based on molality)
∆
H
o
Sol
∆
S
o
Soln
Chem 162 Experiment #4
I. Fractional Crystallization of KNO3 with (NH4)2Fe(SO4)2.6H2O Impurity
II. The Solubility Curve of KNO3
DATA, CALCULATIONS AND GRAPHS
Part I: Fractional Crystallization of KNO3 with (NH4)2Fe(SO4)2.6H2O Impurity
Does the absorbance exceed the desired range (0 to 2.0 A)?
_____yes_______
If so, what dilution factor did you use in order to get the absorbance in the desired range? ____1:10_________
Plot a graph of percent of (NH4)2Fe(SO4)2.6H2O vs.number of crystallization steps (0,1,2) and place it here.
Cover this
instruction box so that your graph is an appropriate size.
Be sure to include a title, axis labels, and use the chart type that connects the data points.
Plot a graph of percent of KNO3 recovered vs. the number of crystallization steps and place it here.
Cover this instruction box so
your graph is appropriately sized.
Your graph should have three points with the first point being 100%.
Be sure to include a title, axis labels, and use the chart type that connects the data points.
II. Determining the Solubility Curve of KNO3
Results and Discussion
1. Do your results indicate that fractional crystallization works?
Why or why not?
The results we obtained from this experiment do support the method of fractional crystallization as a means to isolate pure KNO3. Our impurity
percentages decreased first from 27.92 to 1.423, and then from 1.423 to 0.285 at our final crystallization. This inherently demonstrates the efficiency
of fractional crystallization to isolate impurities from solutions.
2. What is the disadvantage of fractional crystallization?
The dissadvantage of fraction crystallization is that in each crystallization the percent of the final product will undoubtedly decrease. This is because
not every KNO3 molecule will crystallize, therefore losing some of the product. In addition, the final product will not be completely pure because a
small amount of impurity will enter into the crystals. Lastly, fractional crystallization takes a lot of time to get only a relatively small final product.
3. The Handbook of Chemistry and Physics list the enthalpy of solution of KNO3 as 34.89 kJ/mole.
How does your value compare?
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 Spring '08
 N.
 Mole, Solubility, Mass, final product, Crystal, Crystallization

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