Exp 7' Determination of Ksp for a Sparingly Soluble Salt

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Exp 7. Determination of Ksp for a Sparingly
Soluble Salt
Lab 402, 1 student
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• Objective
• To become familiar with equilibria involving
  sparingly soluble
• substances by determining the value of Ksp for
• a sparingly soluble salt.

• Chemicals
• (Each TA should prepare the following solutions
  in advance before the class.)
• 0.0024 M K2CrO4 , 0.004 M AgNO3 , 0.25 M NaNO3
• (0.0024 M K2CrO4- 0.4656 g K2CrO4 per liter)
• (0.0040 M AgNO3- 0.6795 g AgNO3 per liter)
• (0.25M NaNO3- 21.2475 g NaNO3 per liter) X 2

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• Procedure and Apparatus Assignment
• 1) PROCEDURE
• -PART A 4 students, Share the data with your
  partners.
• -PART B One student
• 2) APPARATUS
• A computer with a spectrophotometer per 2
  student
• 100-mL volumetric flask(4) / 4 students
• 12-mL Screw cap tube(3) for centrifuge, conical
  type
• /1 student
• 150-mm test tube (3) rubber stopper(3) / 1
  student
• Cuvette (4) cell holder(1) for spectroscopy/ 1
  student
• Spectrophotometer / 2 students

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How to operate the spectrometer. Your TA will
demonstrate this.
1) Turn on the spectrophotometer and the computer
that runs it. Allow the spectrophotometer to warm
up for 30 minutes before use. Start the program
by double-clicking VisualSpectra2.1-Sr on the
desktop.
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2) Add 0.25 M NaNO3 solution in a cuvette as a
reference. Insert it sample holder. Be sure to
cover sample holder.
3) Move the shutter button toward off. (UV
Visible On)
4) Click the GO button. (Initialize the
detection system and get ready to start.)
5) Click the DARK button . (To obtain a dark
spectrum without a light source.)
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6) Move the shutter button toward ON. (UV
Visible On)
7) Click the refer button. ( To get the
reference spectrum under the light source.)
The ABSORBANCE MODE has been activated.
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8) Click the ABSOR button.
9) Type 375 in the blank of Wavelength.
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10) Fill the cuvette with sample solution. Insert
it into the chamber. Close the sample cover.
(Make sure there are no bubbles in the cuvette.
To remove bubbles, tab the cuvette gently with
your finger.)
11) Click the gtgt button. After a few moments,
the absorbance will be displayed on the
right box. Record the data on your report sheet.
12) Repeat step 10-11 with your remaining
samples.
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Use of Centrifuge

• Use ?/? button to set the value
• RPM-?or ?-2000, Time-?or ?-5 min
• 2) Press a door button.
• 3) Insert the tubes into the chamber
• (Be sure to keep the sample balance.)
• 4) Close the rotor cover, and press the start
  button.
• 5) The door will open automatically after 5 min.

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PART A. Preparation of a Calibration Curve
1. Using a pipette, add 1, 5, 10, and 15mL of
standardized 0.0024M K2CrO4 to each of four
clean, dry 100mL volumetric flasks and dilute to
the 100 mL mark with 0.25 M NaNO3
2. Measure the absorbance of these solutions at
375 nm and plot the absorbance versus
concentration to construct your calibration curve.
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3. Calibration

• Volume of 0.024 M K2CrO4

1 mL of 100mL, 5 mL of 100mL, 10 mL of 100mL, 15
mL of 100mL From these data, we can know the
morality of CrO42-

• For Example,

Abs.
CrO42-(M)
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• Beer-Lambert Law

A abc A absorbance a absorption
coefficient b path length c concentration
Absorbing solution
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• PART B. Determination of the Solubility-Product
  Constant

• Prepare three separate solutions in separate 150
  mm test tubes by adding 5 mL of 0.004 M AgNO3 to
  5 mL of 0.0024 M K2CrO4.
• Stopper each test tube with the rubber
  stopper.

2. Shake the solutions thoroughly at periodic
intervals for about 10 min to establish
equilibrium between the solid phase and the ions
in solution.
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3. Transfer approximately 3 mL of each solution
along with most of the insoluble Ag2CrO4 to the
12-mL centrifuge tubes and centrifuge.
4. Discard the supernatant liquid and retain the
precipitate.
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5. To each of the centrifuge tubes, add 3 mL of
0.25 M NaNO3 , and shake them vigorously for
about 15 min to establish an equilibrium between
the solid and the solution. (If there is not some
solid Ag2CrO4 remaining in the test tubes, start
over again)
6. After shaking the test tubes for about 15 min,
centrifuge the mixtures.
7. Transfer the pale yellow supernatant liquid to
a clean cuvette, and measure the absorbance of
the three solutions.
8. Using your calibration curve, calculate the
molar concentration of CrO42- in each solution.
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• Summary

(Preparation of Calibration Curve)
1. Prepare four 100 mL volumetric flask. 2. Add
1, 5, 10, and 15 mL of 0.0024 M K2CrO4 to four
100 mL volumetric flasks 3. Dilute to the 100 mL
mark with 0.25M AgNO3 4. Measure the absorbance
of five solutions at 375 nm 5. Plot x axis
concentration, y axis absorbance Use
origin program or Excel
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(Determination of the solubility-product const.)
6. Prepare three 150 mm test tubes. 7. Add 5 mL
of 0.004 M AgNO3 to 5 mL 0.0024 M K2CrO4 8. Shake
the solutions and wait for about 15 min. 9.
Transfer 3 mL of each solution along with most of
the insoluble Ag2CrO4 to 12 mL centrifuge tubes
and centrifuge. 10. Discard the supernatant
liquid. 11. Add 3 mL of 0.25 M NaNO3 to each
centrifuge tube. 12. Shake the solutions and
wait for about 15 min and centrifuge 13. Transfer
pale yellow supernatant to a cuvette and measure
the absorbance.
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Waste Disposal Instructions Because chromates
are hazardous, all chromate solutions should be
treated with care. Avoid spilling or touching
these solutions. All excess K2CrO4 solution and
Ag2CrO4 samples from PART A an PART B should be
returned to a specially marked waste
container. Silver nitrate solution is also
hazardous. Any AgNO3 solution that is Spilled on
the skin will cause discoloration after a few
minutes. All excess Ag NO3 solution should be
returned to another specially marked
container. The other solution, NaNO3, also
should be disposed of a designated Waste
container.