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CHEM 403

Exp 8

Experiment 8:

SPECTROSCOPY – USING LIGHT AS A PROBE*

When hydrogen and other gaseous substances are heated, they emit light at a few

characteristic frequencies. Atoms and molecules may also absorb discrete frequencies of light.

Absorption is exactly the opposite of emission. When light is absorbed, an electron undergoes a

transition from a lower (usually the lowest, called the ground state) energy level to a higher energy

level. The atom or molecule gains energy in this process by absorbing a photon of light whose

energy, E, is given by E = ℎν. (where ℎ is Planck’s constant, 6.626 x 10-34 J·s). The energy of the

photon corresponds to the difference in energy between the higher and lower levels. The transition

causes a component of light with a frequency ν to be absorbed while all other frequencies are

transmitted (i.e. not absorbed).

The amount of light absorbed by a substance in solution is determined by the amount of

the substance present, the concentration. The absorption decreases as the concentration decreases.

This statement is expressed mathematically by Beer’s law, A = kc, where A is absorption, c is

concentration, and k is a constant characteristic of the instrument and the substance being studied.

In this experiment, you will receive a solution of cobalt(II) chloride whose concentration

is unknown. Your task is to determine the concentration of this solution.

To begin, the absorbance of a 0.150 M solution of CoCl2 will be measured before and after

a series of dilutions. For each dilution, you will calculate the new concentration of CoCl2 and

prepare a graph plotting the absorbance of each concentration. According to Beer’s Law, the slope

of the best straight line will give you the value of k, the constant from Beer’s Law, under your

experimental conditions.

Your solution of CoCl2 will likely be too concentrated to be reliably measured without

dilution. You must determine the concentration of CoCl2 in this solution from a measurement of

the solution diluted a known amount. It will be up to you to discover the appropriate dilution.

Your measurements will be done on a Spec20™ UV/VIS Spectrophotometer set to a

wavelength of 520 nm. This wavelength is absorbed significantly by CoCl2.

Figure 1. Schematic of a Spectrophotometer

*Adapted from Wentworth, R. A. D. Experiments in General Chemistry.

CHEM 403

Exp 8

When a sample is placed in the spectrophotometer that absorbs at the set wavelength, it

interrupts the transmission of this light from the source to the detector. The detector can read the

difference between the light coming from the source directly and the light coming from the source

that passes through the sample. This difference is given as a readout on the spectrometer.

Procedure

Label seven clean, dry test tubes with numbers 1 to 7. Obtain approximately 30 mL of

0.150 M CoCl2 solution from the instructor. Use a graduated pipet to make the additions of the

CoCl2 solution and distilled water as shown in Table 1. Accurate and consistent pipet work is

crucial for success here. If necessary, practice pipetting exact volumes of water before beginning

to prepare your dilutions. Once the dilutions have been prepared, thoroughly mix the contents of

each tube by shaking it side to side. Do not invert the test tube.

Table 1. Prepared Dilutions of CoCl2

Test tube #

mL 0.150 M CoCl2

mL distilled water

Conc. (M) of CoCl2

1

5.0

0

__________

2

4.0

1.0

__________

3

3.5

1.5

__________

4

3.0

2.0

__________

5

2.5

2.5

__________

6

2.0

3.0

__________

7

1.0

4.0

__________

To calculate the concentration of CoCl2 in each test tube, use Equation 1 to solve for Mdilute

Mconc x Vconc = Mdilute x Vdilute

(1)

where the molarity of the concentrated solution, Mconc, will be 0.150 M for all samples, the volume

of the concentrated solution, Vconc, is the listed volume of the CoCl2 used, the volume of the diluted

solution, Vdilute, for each test tube is 5.0 mL.

To measure the absorbance of your solutions, you want to reduce as much experimental

variation as possible. To that end, you should use the same spectrophotometer for every

measurement you take.

Before measuring one of your solutions, calibrate the photometer. With no cuvette in the

instrument, use the left knob to set the instrument to 0% T (∞A). Place the “blank” cuvette filled

with distilled water in the instrument and line up the mark on the cuvette with the mark on the

photometer. With the “blank” water sample in place, use the right knob to set the instrument to

100% T.

CHEM 403

Exp 8

To measure the absorbance of your sample, fill a cuvette with at least 4 mL of your most

dilute solution (test tube #7). Wipe the outside of the cuvette so that it is free any drops of liquid

or fingerprints. Gently tap the cuvette to remove any bubbles, and place the cuvette in the

instrument so that the mark on the cuvette and the mark on the photometer line up. Record the

absorbance of your sample in your Data & Calculations section.

After measuring this solution, pour the solution back into the appropriate test tube, and

rinse the cuvette with a few drops of your next most dilute solution (test tube #6). Discard this rinse

into a waste beaker, then fill the cuvette with 4 mL of the solution and follow the same steps to

measure the absorbance of this sample.

You do not need to recalibrate the photometer with the “blank” between every

measurement, but should check the calibration after every 3-5 measurements. Each of your

absorbance readings for your solutions should fall between 0.1 and 1.2. See the lab instructor if

your readings are outside of this range.

When you have completed your absorbance readings for all seven of your samples, you

can prepare your graph. Label a piece of graph paper so that absorbance (unitless) appears on the

vertical (y) axis and concentration of CoCl2 (M) appears on the horizontal (x) axis. Use the whole

graph paper – do not squeeze your data points into one corner.

Enter each data point on the graph. The data should correspond roughly to a straight line.

Find the slope, k, by placing a ruler at the origin (x = 0, y = 0) and pivoting until the ruler passes

as close to each point as possible. If there is a significant outlier from the straight line, either ignore

the point, or remeasure the absorbance of that solution. Draw your “best fit” line, and calculate its

slope, k = Δy/Δx, by choosing two points on the line.

Once you have prepared your absorbance versus concentration graph, obtain an unknown

from the instructor. A portion of this solution must be diluted until the absorbance lies within the

absorbance range of your standard solutions run previously. To establish the correct dilution,

measure the absorbance of the undiluted unknown. Use your standard curve to estimate an

appropriate dilution. Using your pipet, prepare that dilution and measure its absorbance. If your

diluted solution is still outside of your known range, return to your concentrated sample, prepare a

more dilute solution, and measure its absorbance.

Calculate the concentration of CoCl2 in the diluted unknown solution using the absorbance

and the “best fit” line on your graph and equation using your calculated k. Then, working

backwards from your diluted concentration, calculate the concentration of CoCl2 in the original

(undiluted) solution.

CHEM 403 – Exp 8

Name:______________________________ Section: _________

Data & Calculations

Preparation of Beer’s Law graph

For your concentrations in Table 1, show one example calculation (for a test tube 2-7):

Absorbances of solutions of known concentrations:

Test tube #

Conc. (M) of CoCl2

Absorbance

1

__________

__________

2

__________

__________

3

__________

__________

4

__________

__________

5

__________

__________

6

__________

__________

7

__________

__________

Select two points that fall close to your line of “best fit” on your prepared graph, and use the

following equation to calculate the slope of your line, which is k in the equation A = kc.

slope, k =

∆y

∆x

k = ________________

(give correct units)

Determination of unknown concentration

Unknown number ______

Trial dilutions (you may not need three trials to obtain an appropriate absorbance)

mL unknown

mL water

Absorbance

___ 5.0____

___ 0.0____

__________

__________

__________

__________

__________

__________

__________

__________

__________

__________

For one of your unknown trials:

Volume of undiluted unknown

__________ mL

Volume of water added

__________ mL

Absorbance = __________

Concentration = __________

You can determine your diluted sample’s concentration two ways. Below, show your calculation

using A = kc, and report the concentration found from the line of best fit on your graph. Do these

values match?

Calculate the undiluted concentration from the dilute concentration:

Undiluted solution concentration = __________

CHEM 403 – Exp 8

Name:______________________________ Section: _________

Prelab Assignment

The following data were collected when the absorbance of a series of solutions containing NiCl 2

were measured.

Test tube #

1

2

3

4

5

5.0

4.0

3.0

2.0

1.0

0.0

1.0

2.0

3.0

4.0

Concentration

_____

_____

_____

_____

_____

Absorbance

1.090

0.855

0.647

0.428

0.215

Volume of 0.390 M NiCl2

solution (mL)

Volume of water (mL)

1. Complete the table by calculating the concentration of NiCl2 in each solution (use Equation 1).

Show one of your calculations below.

2. Select two concentrations and their absorbances, and use the following equation to calculate k

in the equation A = kc. Noting that absorbance is unitless and concentration is in units of M, give

the correct units for k in your answer.

k=

Absorbance1 – Absorbance2

Concentration1 – Concentration2

k = ________________

(give correct units)

3. The absorbance of a solution of NiCl2 of unknown concentration is found to be 0.555. What is

the concentration of this solution based on the equation A = kc?

4. Does it make sense that when preparing a best-fit line for a graph of absorbance versus

concentration, the line should go through the origin, (0,0)? Briefly explain.

…

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