Chemistry: Beer's Law and Absorption Curves

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Questions and Answers

What does Beer’s Law state about the relationship between the concentration of a solution and light absorption?

They are inversely proportional.

They are directly proportional. (correct)

They are unrelated.

They have a non-linear relationship.

What is the formula for Beer’s Law?

A = l/c

A = εlc (correct)

A = ε/c

A = ε + l + c

At what conditions does Beer’s Law not hold true?

At high concentrations only.

At moderate concentrations.

At low concentrations only.

At very high or very low concentrations. (correct)

What can a calibration curve be used for in a spectrophotometry lab?

To determine the concentrations of unknown samples. Signup and view all the answers

How is an absorption curve obtained?

By plotting absorbance against different wavelengths. Signup and view all the answers

What does molar absorptivity (ε) indicate in the context of Beer’s Law?

The degree to which a substance absorbs light. Signup and view all the answers

Which statement about absorbance is true according to Beer’s Law?

Absorbance has no units. Signup and view all the answers

Which factor does NOT affect absorbance in a solution according to Beer’s Law?

Time the solution has been prepared. Signup and view all the answers

What is the proper method for disposing of broken glass test tubes?

Dispose of them in the bucket labeled: ‘Broken glass Only’. Signup and view all the answers

What is the x-axis variable when producing a calibration curve for red dye solutions?

Concentration Signup and view all the answers

If the absorbance of a 5% red dye solution is being analyzed, which type of graph should be produced?

An absorption curve. Signup and view all the answers

What is the formula used to calculate the volume of stock solution required for dilution?

C1V1 = C2V2 Signup and view all the answers

Which of the following concentrations would be expected to have the highest absorbance for the red dye solutions?

10.0% (w/v) Signup and view all the answers

What is the purpose of plotting absorbance against concentration?

To establish a calibration curve. Signup and view all the answers

What safety equipment would be appropriate when handling liquid waste from red dye solutions?

Safety goggles and gloves. Signup and view all the answers

In a laboratory setting, what does the term 'Absorbance' refer to?

The amount of light absorbed by the sample. Signup and view all the answers

What does the '5 mL' signify in the context of this dilutions protocol?

The total volume of each solution Signup and view all the answers

Which piece of equipment is necessary to measure the absorbance of the solutions?

Spectrophotometer Signup and view all the answers

Which of the following is NOT a requirement for the results summary submission?

Include individual submissions for all group members. Signup and view all the answers

What is the process called when reducing the concentration of a solution by adding solvent?

Dilution Signup and view all the answers

What must be done before changing the wavelength on the spectrophotometer?

Blank the spectrophotometer Signup and view all the answers

At which wavelength was the first absorbance value recorded?

400 nm Signup and view all the answers

Which absorbance value corresponds with a wavelength of 470 nm?

600 Signup and view all the answers

What is the increment of wavelength used during the absorbance measurements?

10 nm Signup and view all the answers

How many absorbance values are recorded for the specified wavelength range?

12 Signup and view all the answers

During the calibration standard curve setup, how many concentrations are required to be measured?

10 Signup and view all the answers

What does it mean to 'blank' the spectrophotometer?

To set the spectrophotometer to read zero Signup and view all the answers

What is the maximum wavelength reached during the measurements?

650 nm Signup and view all the answers

What is the first step in preparing the test tubes for spectrophotometry?

Labeling the test tubes with concentrations Signup and view all the answers

What amount of the 10% red dye stock solution should be added to the test tubes?

An amount dependent on the test tube's corresponding concentration Signup and view all the answers

What is the purpose of the BLANK test tube in the experiment?

To serve as a control for zeroing the spectrophotometer Signup and view all the answers

What should be done with the spectrophotometer before measuring absorbance?

The test tubes should be wiped clean Signup and view all the answers

Where should the concentration label be placed on the test tube?

Close to the top of the tube Signup and view all the answers

What is the correct wavelength setting on the spectrophotometer for analyzing the red dye absorbance?

400nm Signup and view all the answers

What must be done with the solutions before placing them in the spectrophotometer?

They should be vortexed thoroughly Signup and view all the answers

Which of the following is NOT a step mentioned in preparing the test tube solutions?

Using a single pipette for all solutions Signup and view all the answers

Study Notes

Beer's Law

Beer’s Law states that the concentration of a solution is directly proportional to the amount of light it absorbs.
This law is expressed as: A = εlc
- A represents absorbance, measured in no units.
- ε represents molar absorptivity, measured in L g-1 cm-1.
- l represents path length, measured in cm.
- c represents the concentration of the compound in the solution, measured in g L-1
Importantly, Beer’s Law is not obeyed at very high or very low concentrations, resulting in a non-linear relationship between absorbance and concentration.

Wavescans (Absorption Curves)

When light of different wavelengths is passed through a coloured solution, some wavelengths are absorbed more than others.
Plotting the amount of light absorbed against different wavelengths creates an ‘absorption’ curve.
This curve allows for the determination of the wavelength that shows the maximum absorbance for the solution.

Calibration Curves (Standard Curves)

A calibration curve is created by plotting absorbance readings against the concentrations of various coloured solutions.
This curve can then be used to determine the concentrations of unknown samples based on their absorbance readings.
The relationship between absorbance and concentration is linear for most coloured solutions that obey Beer’s Law.

Experiment Protocol: Standard Preparation

The experiment involves preparing a set of standard solutions of red dye with known concentrations for a calibration curve: 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, and 10%.
The experiment uses a 10% w/v stock solution of Red Dye #40.
The formula C1V1 = C2V2 is used to calculate the volume of stock solution (V1) needed for each standard concentration (C2) with a total volume (V2) of 5 mL.

Experiment Protocol: Wavescan

The experiment involves measuring the absorbance of a 5% red dye solution at different wavelengths (from 400 nm to 650 nm).
This creates an absorption curve, also known as a wavescan, for the red dye solution.
The spectrophotometer must be blanked with distilled water before each measurement.

Experiment Protocol: Calibration Standard Curve

The experiment involves measuring the absorbance of each prepared standard solution at the wavelength that corresponds to the maximum absorbance from the absorption curve.
These absorbance values are plotted against their corresponding concentrations, creating the calibration curve.
The spectrophotometer must be blanked with distilled water before each measurement.

Experiment Materials

The experiment uses various materials, including:
- 10% w/v stock solution of Red Dye #40
- Distilled water
- Vortex mixer
- Spectrophotometer
- Pipettes (2x 5 mL Mohr or Serological)
- 13x100mm test tubes

Waste Disposal

Liquid waste should be disposed of in the waste container in the fumehood.
Broken glass test tubes should be disposed of in the bucket labeled ‘Broken glass Only’.

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Description

This quiz focuses on Beer's Law and its application in determining the concentration of solutions based on light absorbance. Participants will explore the principles of absorption curves, the significance of calibration curves, and the specifics of light interactions with colored solutions. Test your understanding of these key concepts in chemistry!