Exp 6
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Texas Tech University *
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1307
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Chemistry
Date
Dec 6, 2023
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Uploaded by SuperHumanFerretPerson345
Exp 6: Molarity. Dilutions, and Preparing Solutions
Purpose:
The purpose of this experiment is to become familiar with molarity, the amount of a substance in
a certain volume of solution. Also, to gain experience in the methods of preparing solutions
properly in the laboratory.
Procedure:
A.
Preparing a Stock Solution of Known Concentration (start on page 70): turn on the
spectrophotometer so it has time to heat up before preparing the stock solution. Using a
top-loading balance, obtain 4 g of CuSO4. Determine the mass of a clean 100 mL
volumetric flask and add CuSO4 to the flask with a funnel. Then determine the mass of
the flask and CuSO4. Add 75 mL deionized H2O to the flask and swirl to dissolve the
CuSO4. After all crystals have dissolved, finish filling the volumetric flask to the
calibration mark, then add a cap and swirl to ensure the solution is mixed. Label the
solution “S” and calculate the molarity from the moles of the copper sulfate and volume
of the solution.
C.
Measuring the Absorbance of the Stock Solution (start on page 71): Empty the DI H2O
from the cuvette and rinse several times with several portions of the stock solution. Fill
the cuvette with the stock CuSO4 until it is 80% full and then insert it into the sample
holder of the spectrophotometer. Record the percent transmission. Calculate the
absorbance of the stock solution using equation 4 on page 69. Remove the cuvette and
empty the contents into a waste beaker.
E.
Construction of a Calibration Graph (bottom of page 71): a plot of absorbance (A)
versus molarity (M) will be prepared to construct a calibration graph. Five solutions will
be used, the stock solution and four dilutions as data points. Obtain and clean four 50
mL volumetric flasks and label them A, B, C, D. Using appropriate pipets and stock
solution, add 5 mL to volumetric flask A, 10 mL to volumetric flask B, 15 mL to
volumetric flask C, and 20 mL to volumetric flask D. Dilute each flask to the calibration
mark using DI H2O, and cover with a cap and mix by swirling for a minute. Rinse the
cuvette with solution D and measure the percent transmission and repeat for solutions C,
D, and A. Calculate the molarity of each solution and calculate the absorbance of each
using equation 4 on page 69. Prepare a plot of absorbance versus molarity using the data
points, including the trend line equation for a linear fit. (If sharing a spectrophotometer,
be sure to re-zero the instrument with DI H2O in your cuvette before each use.)
H.
Determining the Concentration of an Unknown Solution (bottom of page 72): obtain an
unknown solution and label it with the ID given to you by your instructor. Measure the
percent transmission of the unknown solution. Determine the molarity of the solution using
the equation obtained from the calibration graph and the absorbance calculated from the
measured percent transmission (experiment part on page 71).
Clean and rinse all glassware
thoroughly.
Waste Disposal Instructions:
Dispose of all waste as indicated by your instructor.
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