Concentration of a Solution (Beers Law) 012-10736 r1.04.

Concentration of a Solution (Beer’s Law) 012-10736 r1.04

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IntroductionJournals and Snapshots

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Concentration of a Solution (Beer’s Law)

Lab Challenge

The more concentrated (darker) a colored solution is the more light the solution is absorbing. Given the mathematical relationship between the absorbance of light and the concentration of a solute, determine the concentration of a copper(II) sulfate solution.

Copper(II) sulfate solutions


Background

• Light is a form of energy that can interact with chemical substances. • By examining the wavelength of light (its color) that is absorbed by a chemical

substance, the chemicals identity and quantity can be determined.


Light reflected

Light absorbed

...Background

• How a chemical appears to our eyes depends on how light interacts with the substance.

• Light can be transmitted (passed through), reflected (scattered), or absorbed by a substance.

Light transmitted

Light absorbed


Self-Check

1. The green light in the diagram is being ____________.

a) reflected

b) absorbed

c) transmitted

d) transmogrified


• the chemical nature of the substance.

NiCl2Co(NO3)2

K2Cr2O7 K2CrO4 CuSO4 KMnO4

...Background

The color observed depends on:

• the thickness of the sample the light passes through.

• the concentration of the substance in solution.

Concentrated

Dilute

20 mm path length

55 mm path length


...Background• These variables are mathematically related to one another in Beer's law:

A = ε × b × c

A = measured absorbance of the light

ε = absorbance coefficient (chemical nature of the substance)

b = path distance (thickness of the sample the light passes through)

c = concentration of solute

• This equation is a direct relationship (as c increases, A increases) supporting the observation that more concentrated solutions absorb more light and hence look darker.


Self-Check

2. According to Beer's Law, when you decrease concentration c for a solution, what should result?

a) Absorbance constant is canceled out

b) Distance stays the same

c) Absorption increases

d) Distance increases

e) Absorption decreases

Beer's Law

A = ε × b × c


...BackgroundIn this experiment, a colorimeter will be used to find the mathematical relationship between the absorbance of orange light (λ=610 nm) and the concentration of copper(II) sulfate.

Light Source


• Since the colorimeter has a fixed path length b, and the experiment only uses copper(II) sulfate (ε) Beer's law (A = ε × b × c ) can be simplified to:

• The direct relationship between absorbance and concentration is mathematically represented by a straight line: Y = mX + b.

• The mathematical relationship (equation) will be determined by graphing measured absorbencies at known concentrations of copper(II) sulfate and fitting the data to a line (calibration curve).

• The equation of the linear fit will be used to determine the concentration of an "unknown" copper(II) sulfate solution.

Y = mX + b

A = constant × c


...Background

Safety

• Follow all common laboratory safety procedures.

• Wash your hands with soap and water after handling the solutions, glassware, and equipment.

• Copper(II) sulfate is hazardous to the environment and should not be disposed of down the drain. Make sure you follow your teacher’s instructions on how to properly dispose of the copper(II) sulfate solutions.

BE SAFEAlways wash hands to remove residue

before leaving


Materials and EquipmentCollect all of these materials before beginning the lab.

• Colorimeter

• Sensor extension cable

• Glass cuvettes with caps (7)

• Test tubes (6), 20-mm x 150-mm

• Test tube rack

• Beakers (2), 100-mL


Materials and EquipmentAlso collect these additional materials before beginning.

• Volumetric pipettes (2), 10-mL

• Pipette bulb or pump

• Non-abrasive cleaning tissue

• 0.80 M copper(II) sulfate solution, 30 mL

• Unknown copper(II) sulfate solution, 10 mL

• Distilled (deionized) water, 30 mL


Distilled Water

Sequencing Challenge

A. Determine the linear equation for the calibration curve's line of best fit.

B. Create a calibration curve by graphing absorbance versus concentration.

C. Measure the absorbance of the unknown and determine its concentration using the equation.

The steps to the left are part of the procedure for this lab activity. They are not in the right order. Determine the correct sequence of the steps, then take a snapshot of this page.

D. Create five copper(II) sulfate solutions of known concentration and measure the absorbance of each.


1. Connect the colorimeter to the data collection system using a sensor extension cable.

2. Calibrate the colorimeter so that the colorimeter reads an absorbance of 0.0 for distilled water (the solvent).

a. Fill a cuvette with distilled water and cap it.b. Holding the cuvette by the lid, wipe the outside of

the cuvette with a non-abrasive cleaning tissue.c. Place the cuvette in the colorimeter and close the lid.d. Push the green calibrate button on the colorimeter.e. When the green light turns off, the calibration is

complete and the cuvette can be removed from the colorimeter.

Setup: Calibration Curve

Calibrate Button


Setup: Calibration CurveQ1: Why is it necessary to calibrate the

colorimeter using distilled water?



3. Measure approximately 30 mL of 0.80 M copper(II) sulfate stock solution into a 100-mL beaker.

4. Measure approximately 30 mL of distilled water into a different 100-mL beaker.

5. Take six clean, dry test tubes and place them in a test tube rack. Label the test tubes: 1, 2, 3, 4, stock, and unknown.

Q2: Why do the test tubes need to be dry? What error would be caused by wet test tubes?



6. Prepare the five standard copper(II) sulfate (CuSO4) solutions listed in the table below. Use one volumetric pipette for the CuSO4 and a different volumetric pipette for the water (H2O).

7. Thoroughly mix each solution by gently swirling each test tube.

stock

1

2

3

4

2.0 mL

4.0 mL

6.0 mL

8.0 mL

10.0 mL

8.0 mL

6.0 mL

4.0 mL

2.0 mL

0 mL

0.16

0.32

0.48

0.64

0.80

Test Tube # 0.80 M CuSO4 H2O Concentration (M)


Q3: What will the colorimeter measure? Is this the independent or dependent variable?


8. Fill one cuvette with the 0.16 M CuSO4 solution and cap it. Label the top of the cuvette lid.

9. Continue to fill one cuvette at a time until each solution is in a cuvette, the cap is on, and it is clearly labeled (0.32 M, 0.48 M, 0.64 M, & 0.80 M).


Collect Data: Calibration Curve

1. Use a non-abrasive cleaning tissue to wipe the outside of the cuvette containing 0.16 M CuSO4 and place the clean cuvette inside the colorimeter.

2. Close the lid of the colorimeter.

Q4: Why is it necessary to wipe the outside of the cuvette before you place it in the colorimeter?

Q5: Why is it necessary to close the lid of the colorimeter before recording the absorbance?


*To Enter Data into a Table: 1. Tap to open the tool

palette.2. Tap then tap a cell in the

data table to highlight it in yellow.

3. Tap to open the Keyboard screen.

3. Tap to start a data set.4. Enter the molarity of the first sample in the appropriately labeled column.*5. Tap to record the

absorbance of this solution.


6. Remove the 0.16 M cuvette.

7. Wipe the cuvette of the next sample and place it in the colorimeter.

8. Enter the molarity of this sample into the table.

9. Tap to record the absorbance of this solution.

10. Repeat this process for the last three solutions.

11. Tap to stop the collecting of data in this set.


*To Apply a Curve Fit: 1. Tap to open the tool

palette.2. Tap to open the Curve

Fit screen. 3. Tap the name of the curve

fit required.

Data Analysis: 1. Create a linear best fit line

of the data collected*.Note: the equation of the best fit line gives:

A = constant × cy = m × x


Note: SPARK will use this equation to calculate the concentration of your unknown.

2. What is the equation for the line of best fit? Replace y with absorbance (orange 610) and x with concentration.

3. Solve the equation for concentration.

4. Enter the equation you determined in question #3 into the data collection system calculator.*

*To create a Calculation:1. Tap to open the

Experiment Tools screen.2. Tap CALCULATED DATA to

open the calculator.3. Enter the calculation in the

space provided.4. Tap Measurements to insert

collected data into the calculation.


Setup: Determining Concentration1. Obtain 10 mL of the unknown copper (II) sulfate solution from your

instructor. Put this solution in the test tube labeled "Unknown Concentration".

2. Fill a dry, clean cuvette with the unknown solution, cap it, and label it "unknown".

3. Wipe the outside of the cuvette with a non-abrasive cleaning tissue and place the cuvette in the colorimeter.

4. Close the lid of the colorimeter.



*To Change the variable on a digits display:

1. Tap to open the tools palette.

2. Tap to open the Digits Properties screen.

3. Tap the Measurement box and select the desired variable.

Collect Data:1. Change one of the digits

displays below to show the calculated concentration of your unknown solution.*

2. Tap to start a data set.

3. Tap to stop the data set.

4. Enter the absorbance and the concentration of the unknown solution in the text box on the top right.

Analysis1. State Beer's law. Does your data support this statement?


Analysis2. Would an error occur if some of the

cuvettes were dirty? Explain.


Analysis3. Explain the difference between “absorbance” and

“transmittance” of light.


Analysis4. Why is CuSO4 a bluish color? Are colors of light

other than blue transmitted?


Analysis5. Why was the absorbance of orange

(610 nm) light used instead of another color?


Synthesis1a. A solution of sodium sulfate is clear and colorless, and yet a solution of copper(II)

sulfate is blue. Which ions are causing the blue color?

1b. Could you use Beer's law to find the concentration of a sodium sulfate solution?


Synthesis2a. What is the minimum number of points needed to create a calibration curve?

2b. How many points were used in this experiment? Why?


Synthesis3. Other sensors, such as for pH and conductivity, need to be calibrated, but are

able to determine unknown concentrations without creating a calibration curve. Explain how this works.


Multiple Choice1. Which of the following variables affects the

absorbance of light in a solution?

a) The distance the light has to travel through the solution (path length.

b) The amount of solute in each volume (concentration).

c) The wavelength of the light that is interacting with the solution.

d) All of the above variables affect the absorbance of light in a solution.


Multiple Choice 2. A sample 0.10 M nickel (II) chloride is placed into

a cuvette with a 1.00 cm path length. The solution has a measured absorbance of 2.0. What would you expect the absorbance of a 0.05 M nickel (II) chloride solution to be?

a) 1.0

b) 2.0

c) 4.0

d) not enough information

NiCl2 solution


Multiple Choice3. What should be in the ‘blank’ cuvette when the

colorimeter is calibrated?

a) nothing

b) the solution with the greatest concentration of solute

c) the solvent

d) a 1.0 M sample of the solution


??

Multiple Choice4. What color of light is transmitted through a

copper(II) sulfate solution?

a) red

b) orange

c) green

d) blue


Multiple Choice5. How many colors make up white light?

a) 1

b) 3

c) 4

d) more than five

e) none


You have completed the lab.Congratulations!

Please remember to follow your teacher's instructions for cleaning-up and submitting your lab.


All images were taken from PASCO documentation, public domain clip art, or Wikimedia Foundation Commons.

1. LIGHT BULB http://www.freeclipartnow.com/household/lightning/Lightbulb3.jpg.html

2. BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.htm

3. COLORED LIQUIDS http://commons.wikimedia.org/wiki/File:Coloured-transition-metal-solutions.jpg

4. VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html

5. COLOR SPECTRUM http://commons.wikimedia.org/wiki/File:EM_Spectrum3-new.jpg

6. EYE http://freeclipartnow.com/people/body-parts/eyes/big-blue-eye.jpg.html10.

7. BEAKER http://www.freeclipartnow.com/science/flasks-tubes/beaker-2.jpg.html

8. BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html

ReferencesConcentration of a Solution (Beer’s Law)

Concentration of a Solution (Beers Law) 012-10736 r1.04.

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