Dilution
2003 Required D
Information Given
• A student is instructed to determine the
concentration of a solution of CoCl2 based
on absorption of light (spectrometric/
colormetric method). The student is
provided with a .1 M solution of CoCl2 with
which to prepare standard solutions with
concentration of .02 M, .04 M, .06 M and
.08 M.
Questions….
• Part A:
– Describe the procedure for diluting the
.1 M solution to a concentration of .02 M
using distilled water, a 100 mL
volumetric flask, and a pipet or buret.
Include specific amounts where
appropriate.
Dilution: How To
• To dilute:
• You are changing the concentration but keeping the
same substance of a solution. Therefore, you should
just focus on the units and concentration to work out the
problem as well as moles.
» Remember!!!!
M1V1  M 2V2
M stands for concentration of solution, M1 is the initial while M2 is how
much you want. V1 is the original volume while V2 is the new amount
needed, remembered this is the total new amount not the amount
added
Dilution
.1 _ mole
.02 _ mole
(
)  ( x _ liters)  (
) * (.1 _ liter )
1 _ liter
1 _ liter
Then you would solve for x. This would be the
amount of liters that you need of the .1 M solution.
Then once gaining this amount of .02 liters or 20
mL, this is the amount of .1 M of CoCl2 you need to
first put into the volumetric flask. After you fill this
up with 80 mL of water it would be the total to fill up
the volumetric flask.
Questions…..
• Part B:
– Identify the optimum wavelength for the
analysis.
• Optimal wavelength is the maximum absorbance
at a wavelength (self explanatory). Looking at the
graph, it is clear to see that the maximum
absorbance is at about 510 nm.
Questions….
• Part C:
– The absorbance of the unknown solution is
.275. What is the concentration of the
solution?
• Use the graph to determine the concentration. The
concentration would be .05 M.
Questions….
• Part D:
– Beer’s Law is an expression that includes
three factors that determines the amount of
light that passes through a solution. Identify
two of these factors.
– What is Beer’s Law?
» determines the quantitatively analysis using spectroscopy. Spectroscopy
studies em radiation emitted or absorbed by chemical species.
Beer-Lambert Law

l
c
A  abc /_ A   * l * c
Is molar absorptivity or molar extinction coefficient (L/mol*cm)
Length of path, distance light travels through the solution
Concentration of solution (M)
Questions…
• Therefore three of the factors are just the
components of the equation for Beer’s
Law.
» molar absorptivity or molar extinction coefficient
(L/mol*cm)
» Path length, distance light travels through solution
» Concentration of absorbing species
• Part E:
Questions….
– The student handles the sample container
that holds the unknown solution and leaves
fingerprints in the path of the light beam. How
will this affect the calculated concentration of
the unknown? Explain your answer.
• The fingerprints will not allow the light to go
through and distort it somewhat. Therefore the light
would be scattered making the detector get less
light. This would increase the absorbance making
an increased concentration from Beer’s Law.
Questions…
• Absorbance
• Is defined as base 10 log of the reciprocal of the
transmittance of a pure solvent.
I-intensity of light at specified wavelength
I0- intensity of light before it enters the sample
I/I0- -transmittance
Questions….
• Part F:
– Why is this method of determining the
concentration of CoCl2 whereas using the
same method for measuring the concentration
of NaCl would not be appropriate?
» Na+ doesn’t absorb energy in the visible spectrum,
but Co2+ does its pink colored.
NaCl-clear
CoCl2 -pink
Wrap Up
• For this problem:
– Dilution
– Beer’s Law
– Reading Graphs
– Some solution color information