Transcript Beer’s Law - Schoolwires

BEER’S LAW
AP Free Response Questions
2003
Beer’s Law
A student is instructed to determine the concentration is a
solution of CoCl2 based on the absorption of light.
(spectrophotometer/colorimeter method).The student is
provided with a 0.10M CoCl2 solution with which they are to
prepare standard solutions with concentrations of 0.020M,
0.040M, 0.060M and 0.080M.
Describe the procedure for diluting the 0.10M solution to a
concentration of 0.020M using distilled water, a 100ml
volumetric flask, and a pipet or buret. Include Specific
amounts where appropriate.
Solution: 1pt for CoCl2; 1 pt for H2O
M1V1 = M2V2
0.10M(V1) = 0.02M(100ml)
V1 = 20ml CoCl2
80ml = H2O
Pipet 20ml of the CoCl2 into a volumetric flask. Add enough
distilled water to the flask to reach the 100ml marking on
the flask. Mix well.
Part b.
A student takes the 0.10M solution and determines the
percent transmittance and the absorbance at various
wavelengths. The two graphs below represent the data:
b. Identify the optimum wavelength for the analysis.
Solution – 1 point: The optimum wavelength is in the range
of 490 – 520nm. The actual wavelength is 510nm
Part c
The student measure the absorbance of the 0.020M, 0.040M,
0.060M, 0.080M and 0.10M solutions. The data are plotted
below.
c. The absorbance of the unknown solution is 0.275. What is the
concentration of the solution?
Solution – 1 point: The acceptable range is 0.045M – 0.055M.
The actual value is 0.050M.
d. Beer’s Law is an expression that
includes three factors that determine the
amount of light that passes through a
solution. Identify two of these factors.
Solution: 1 point each factor ID’d and
explained
A=abc
a=molar absorbtivity
b=path length of the cuvette
c=concentration
Parts e.
e. The student handles the sample container (cuvette) 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.
Solution e.
The presence of fingerprints will scatter or absorb light.
Since less light reaches the detector, the solutions will have
a higher apparent absorbance (1 point), and therefore a
higher reported concentration.(1 point)
Part f
f. Why is this method of determining the concentration of
CoCl2 solution appropriate, whereas using the same
method for measuring the concentration of NaCl solution
would not be appropriate?
Solution
f. CoCl2 is a colored solution with will absorb visible light (1
point) and NaCl is a colorless solution and will not absorb
visible light (1 point).