Serial Dilution & Spectronic-20 Activity Name: _____________________
Date: ___________ Hour: _____
Acids and bases usually come in concentrated form, as “concentrates,” but they
are rarely used in this form. In order to obtain a certain concentration of acid or base,
dilutions are made. A dilution is prepared by adding a specific amount of a concentrate
to water to obtain a new volume and concentration. One common unit of concentration
is molarity (M), which is equal to moles of solute per liters of solution. In order to
calculate dilutions of solutions the equation:
MV = MV 1122
where M is concentration in molarity (mol solute/L solution) and V is volume (L), is used.
Spectroscopy is one method of determining the concentration of an unknown
solution. By measuring the respective absorbance values for solutions with known
concentrations, a calibration curve can be constructed. The absorbance of an
unknown can be used to determine its concentration through use of this calibration
curve. In this lab we will use a Spectronic-20 to measure the concentration of solutions
in order to construct a calibration curve for the diluted solutions prepared. Finally, an
unknown solution will be analyzed to determine its concentration using the calibration
Unknown Letter: _____ Matching Cuvette/Dilution #: _____
Wavelength used: ________ nm
Cuvette/Dilution # Absorbance
Procedure: Part 1: Dilutions and Eye Analysis
? Students will pair up into groups of four and obtain a test tube rack containing 9
cuvettes, a test tube rack, a pipet, a pipet bulb, a beaker, and one cuvette filled with
10.00 mL of simulated nitric acid.
CAUTION: This is not actual nitric acid so it will not burn you; however, it is not
food coloring, but a laundry dye so it WILL stain you and your clothes. Please be
careful when transferring and transporting solutions.
? To each of the 9 empty cuvettes students should add 3.00 mL of water from a
beaker using a pipet and bulb.
? We will be diluting the original acid solution by a factor of 2 into each of the
remaining 9 cuvettes. The group should begin by transferring 3.00 mL of the
contents of cuvette 1 into cuvette 2 using the pipet and bulb.
? Once the solution in cuvette 2 has been mixed thoroughly, as shown by the teacher,
the group should transfer 5.00 mL of the solution in cuvette 2 to cuvette 3 using the
? This procedure is repeated until all of the dilutions are complete.
? Each group will then obtain an unknown solution of simulated nitric acid.
? The letter of the unknown should be recorded and the group should decide which
cuvette (1-10) the unknown matches most closely and record its number.
Part 2: Spectronic-20 Analysis
? Each group will use the Spectronic-20 instrument to measure the absorbance of the
solutions from part one. The Spectronic-20 should be set at a wavelength of _____
nm for these measurements.
? First, students should measure a blank solution (just water) and set the absorbance
measurement at zero to calibrate the instrument.
? Each cuvette should be measured and their absorbance values should be recorded.
Later these values will be used to prepare a calibration plot of absorbance vs.
? The absorbance should be measured and recorded for the unknown solution used in
Part 1. Make sure you have recorded the unknown letter.
? The dilution number and concentration of the unknown will be determined based on
the previously constructed calibration curve.
Post-Lab Questions: Name: _____________________
Date: ___________ Hour: _____
1. What is the name of the point at which you measure the volume of a solution in a
pipet or graduated cylinder?
2. If the initial concentration of your acid in cuvette 1 was 15 M, what was the nd cuvette? What was the concentration in concentration of the solution in the 2ththe 5 cuvette?
3. Why do you need to calibrate the Spectronic-20 with a blank first?
4. If I forgot to wipe the cuvette with a Kim Wipe and there were fingerprints of the
glass, how could this affect the value of the absorbance read by the
spectrophotometer? Would it increase or decrease the absorbance value?
5. On the graph paper provided, plot the absorbance vs. the cuvette/dilution number
for each of the solutions measured. Start with the most dilute solution near the y-
axis as shown in class. From this plot, determine the cuvette/dilution number of
the unknown solution and label the point on your graph.
Unknown Letter: ______ Matching Cuvette/Dilution # from graph: _____
Explain how you found this.
6. Calculate the concentration of the unknown cuvette assuming that the original
acid solution had a concentration of 15 M. (Show your work for full credit)
7. What volume of the original acid solution (15 M) would you need to use to make
a 20.00 mL solution with a concentration of 4 M? (Show your work for full credit)
8. Explain the procedure you would use to prepare the solution in question 7 using
the techniques learned in lab.