b.
Discard the cuvette contents as directed. Rinse the cuvette twice with the Test Tube 2
solution, fill the cuvette 3/4 full, and place it in the device. After the reading stabilizes,
click
, type
2
in the edit box, and click
.
c.
Repeat the Step b procedure to find the absorbance of the solutions in Test Tubes 3, 4,
and 5 (the standard solution).
d.
From the table, record the absorbance values for each of the five trials in your data table.
e.
Dispose of all solutions as directed by your instructor.
PROCESSING THE DATA
1. Write the
K
c
expression for the reaction in the Data and Calculation table.
2.
Calculate the initial concentration of Fe
3+
, based on the dilution that results from adding
KSCN solution and water to the original 0.0020 M Fe(NO
3
)
3
solution. See Step 2 of the
procedure for the volume of each substance used in Trials 14. Calculate [Fe
3+
]
i
using the
equation (will be the same for all four test tubes):
[Fe
3+
]
i
=
Fe(NO
3
)
3
mL
total mL
!
(0.0020 M)
3.
Calculate the initial concentration of SCN
–
, based on its dilution by Fe(NO
3
)
3
and water:
[SCN
–
]
i
=
KSCN mL
total mL
!
(0.0020 M)
In Test Tube 1, [SCN
–
]
i
= (2 mL / 10 mL)(0.0020 M) = 0.00040 M. Calculate this for the
other three test tubes.
4. [FeSCN
2+
]
eq
is calculated using the formula:
[FeSCN
2+
]
eq
=
A
eq
A
std
!
[FeSCN
2+
]
std
where
A
eq
and
A
std
are the absorbance values for the equilibrium and standard test tubes,
respectively, and [FeSCN
2+
]
std
= (1/10)(0.0020) = 0.00020 M. Calculate [FeSCN
2+
]
eq
for
each of the four trials.
5. [Fe
3+
]
eq
: Calculate the concentration of Fe
3+
at equilibrium for Trials 14 using the equation:
[Fe
3+
]
eq
= [Fe
3+
]
i
– [FeSCN
2+
]
eq
6. [SCN
–
]
eq
: Calculate the concentration of SCN

at equilibrium for Trials 14 using the
equation:
[SCN
–
]
eq
= [SCN
–
]
i
– [FeSCN
2+
]
eq
7. Calculate
K
c
for Trials 14. Be sure to show the
K
c
expression and the values substituted in
for each of these calculations.
8. Using your four calculated
K
c
values, determine an average value for
K
c
.
How constant were
your
K
c
values?
Page 73 of 77
Chemical Equilibrium: Finding a Constant, K
c
DATA AND CALCULATIONS
Absorbance
Trial 1
_______
Trial 2
_______
Trial 3
_______
Trial 4
_______
Absorbance of standard (Trial 5)
_______
Temperature
_______ °C
K
c
expression
K
c
=
[Fe
3+
]
i
[SCN
–
]
i
[FeSCN
2+
]
eq
[Fe
3+
]
eq
[SCN
–
]
eq
K
c
value
Average of
K
c
values
K
c
= ________
at ________°C
Page 74 of 77
Lab
16
The Determination of the
Percent Water in a Compound
The polarity of the water molecule, which makes it a great solvent for ionic compounds, causes
water molecules to cling to the structure of solid substances. When this occurs, the trapped water
molecules are called water of hydration and they become an integral part of the crystal structure.
There are many compounds that have a tendency to absorb water vapor from the air. These
compounds are said to be
hygroscopic
, and can be used as moisturereducing agents. Other
compounds absorb such large quantities of water vapor that they will actually dissolve in their
own water of hydration, a property known as
deliquescence
.
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