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Questions and Answers
A spectrophotometer measures the transmittance ($T$) of a solution as 0.25. What is the absorbance ($A$) of this solution?
A spectrophotometer measures the transmittance ($T$) of a solution as 0.25. What is the absorbance ($A$) of this solution?
- 2.0
- 0.602 (correct)
- 1.301
- 0.398
A solution has an absorbance of 1.0. If the path length of the cuvette is doubled, what will be the new absorbance, assuming the concentration remains constant?
A solution has an absorbance of 1.0. If the path length of the cuvette is doubled, what will be the new absorbance, assuming the concentration remains constant?
- 2.0 (correct)
- 1.0
- 4.0
- 0.5
Which of the following changes will always lead to a linear change in the measured absorbance, assuming the Beer-Lambert law is obeyed?
Which of the following changes will always lead to a linear change in the measured absorbance, assuming the Beer-Lambert law is obeyed?
- Linear increase in concentration (correct)
- Exponential increase in concentration
- Square root increase in concentration
- Logarithmic decrease in path length
A colored solution shows 50% transmittance. If the concentration of the solution is doubled, what will be the approximate transmittance?
A colored solution shows 50% transmittance. If the concentration of the solution is doubled, what will be the approximate transmittance?
The molar absorptivity of a substance at 500 nm is $1.5 \times 10^4 M^{-1}cm^{-1}$. What concentration of the substance will give an absorbance of 0.75 in a 1 cm path length?
The molar absorptivity of a substance at 500 nm is $1.5 \times 10^4 M^{-1}cm^{-1}$. What concentration of the substance will give an absorbance of 0.75 in a 1 cm path length?
What is the relationship between the intensity of incident light ($I_0$) and transmitted light ($I$) when a sample does not absorb light at a particular wavelength?
What is the relationship between the intensity of incident light ($I_0$) and transmitted light ($I$) when a sample does not absorb light at a particular wavelength?
According to Lambert-Beer's Law, what parameters does absorbance (A) depend on?
According to Lambert-Beer's Law, what parameters does absorbance (A) depend on?
In Lambert-Beer's Law, what does the molar absorption coefficient ($\epsilon$) represent?
In Lambert-Beer's Law, what does the molar absorption coefficient ($\epsilon$) represent?
What happens to the wavelength of light upon absorption by a sample, according to the information provided?
What happens to the wavelength of light upon absorption by a sample, according to the information provided?
Lambert-Beer's Law describes the relationship between the decrease in intensity of radiation and which of the following?
Lambert-Beer's Law describes the relationship between the decrease in intensity of radiation and which of the following?
If the concentration of a solution is doubled, and assuming all other variables remain constant, what happens to the absorbance, according to Beer's Law?
If the concentration of a solution is doubled, and assuming all other variables remain constant, what happens to the absorbance, according to Beer's Law?
A chemist measures the absorbance of a solution using a spectrophotometer. If the reading shows a very high absorbance value, what can be inferred about the transmitted light intensity?
A chemist measures the absorbance of a solution using a spectrophotometer. If the reading shows a very high absorbance value, what can be inferred about the transmitted light intensity?
Consider two solutions with the same concentration and molar absorption coefficient. Solution A is in a cuvette with a path length of 1 cm, and Solution B is in a cuvette with a path length of 2 cm. How will the absorbance values compare?
Consider two solutions with the same concentration and molar absorption coefficient. Solution A is in a cuvette with a path length of 1 cm, and Solution B is in a cuvette with a path length of 2 cm. How will the absorbance values compare?
Which of the following is NOT a characteristic of the molar absorption coefficient according to the Beer-Lambert Law?
Which of the following is NOT a characteristic of the molar absorption coefficient according to the Beer-Lambert Law?
A scientist observes that a solute in their experiment undergoes significant polymerization. What is the most likely consequence regarding the applicability of the Beer-Lambert Law?
A scientist observes that a solute in their experiment undergoes significant polymerization. What is the most likely consequence regarding the applicability of the Beer-Lambert Law?
In applying the Beer-Lambert Law to determine the concentration of an unknown substance, which factor must be known or determined independently?
In applying the Beer-Lambert Law to determine the concentration of an unknown substance, which factor must be known or determined independently?
A researcher is using a spectrophotometer with a polychromatic light source. What adjustment should they make to ensure the Beer-Lambert Law is applied correctly?
A researcher is using a spectrophotometer with a polychromatic light source. What adjustment should they make to ensure the Beer-Lambert Law is applied correctly?
A solution deviates from Beer-Lambert Law due to high concentration. Which of the following is the most effective method to rectify this issue and obtain reliable measurements?
A solution deviates from Beer-Lambert Law due to high concentration. Which of the following is the most effective method to rectify this issue and obtain reliable measurements?
According to the Lambert-Beer Law, what is the relationship between the absorbance (A) of a solution and its concentration (c) when path length (l) and molar absorptivity ($\epsilon$) are constant?
According to the Lambert-Beer Law, what is the relationship between the absorbance (A) of a solution and its concentration (c) when path length (l) and molar absorptivity ($\epsilon$) are constant?
In the Lambert-Beer Law, what does the term 'molar absorptivity' ($\epsilon$) represent?
In the Lambert-Beer Law, what does the term 'molar absorptivity' ($\epsilon$) represent?
A monochromatic light beam passes through a solution. If the concentration of the solution is doubled, what happens to the absorbance, assuming the path length remains constant?
A monochromatic light beam passes through a solution. If the concentration of the solution is doubled, what happens to the absorbance, assuming the path length remains constant?
The intensity of a light beam decreases as it passes through a solution. What factors, as described by the provided content, influence this decrease?
The intensity of a light beam decreases as it passes through a solution. What factors, as described by the provided content, influence this decrease?
What is the significance of the negative sign in the differential form of Lambert-Beer's Law, $-dI/dx = KcI$?
What is the significance of the negative sign in the differential form of Lambert-Beer's Law, $-dI/dx = KcI$?
If the transmittance (T = I/I0) of a solution is 0.1, what is the absorbance (A)?
If the transmittance (T = I/I0) of a solution is 0.1, what is the absorbance (A)?
A solution has an absorbance of 0.5 at a specific wavelength with a path length of 1 cm. If the molar absorptivity at that wavelength is 250 $L \cdot mol^{-1} \cdot cm^{-1}$, what is the concentration of the solution?
A solution has an absorbance of 0.5 at a specific wavelength with a path length of 1 cm. If the molar absorptivity at that wavelength is 250 $L \cdot mol^{-1} \cdot cm^{-1}$, what is the concentration of the solution?
A researcher measures the absorbance of a sample and finds it to be higher than expected. Which of the following is least likely to be a cause for concern regarding the accuracy of the absorbance reading?
A researcher measures the absorbance of a sample and finds it to be higher than expected. Which of the following is least likely to be a cause for concern regarding the accuracy of the absorbance reading?
A solution has 20% transmittance. What is its absorbance?
A solution has 20% transmittance. What is its absorbance?
If the absorbance of a solution is 1, what is the percent transmittance (%T)?
If the absorbance of a solution is 1, what is the percent transmittance (%T)?
A compound in a 2 cm cell exhibits an absorbance of 0.5. If the molar absorption coefficient is 250 $L \cdot mol^{-1} \cdot cm^{-1}$, what is the concentration of the compound?
A compound in a 2 cm cell exhibits an absorbance of 0.5. If the molar absorption coefficient is 250 $L \cdot mol^{-1} \cdot cm^{-1}$, what is the concentration of the compound?
Which change will increase the absorbance of a solution, assuming Beer's Law is obeyed?
Which change will increase the absorbance of a solution, assuming Beer's Law is obeyed?
What are the units of the molar absorption coefficient ($\epsilon$) in Beer's Law?
What are the units of the molar absorption coefficient ($\epsilon$) in Beer's Law?
A solution with an initial transmittance of 50% has its concentration doubled. Assuming Beer's Law applies, what is the new approximate transmittance?
A solution with an initial transmittance of 50% has its concentration doubled. Assuming Beer's Law applies, what is the new approximate transmittance?
Which of the following correctly expresses the relationship between transmittance (T) and absorbance (A)?
Which of the following correctly expresses the relationship between transmittance (T) and absorbance (A)?
A Beer's Law plot deviates from linearity at high concentrations. What is a likely reason for this deviation?
A Beer's Law plot deviates from linearity at high concentrations. What is a likely reason for this deviation?
Flashcards
Absorption of Light
Absorption of Light
The decrease in light intensity as it passes through a substance.
No Absorption (Light)
No Absorption (Light)
If a sample doesn't absorb light, incident and transmitted intensities are equal.
Absorption Occurs (Light)
Absorption Occurs (Light)
If a sample absorbs light, transmitted intensity is less than incident intensity.
Light Absorption Properties
Light Absorption Properties
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Lambert-Beer Law Formula
Lambert-Beer Law Formula
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Lambert-Beer Law Definition
Lambert-Beer Law Definition
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Absorbance (A)
Absorbance (A)
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Molar Absorption Coefficient (ε)
Molar Absorption Coefficient (ε)
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Path Length (l)
Path Length (l)
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Transmittance (T)
Transmittance (T)
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Percent Transmittance (%T)
Percent Transmittance (%T)
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Molar Absorption Coefficient Properties
Molar Absorption Coefficient Properties
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Using Beer-Lambert Law
Using Beer-Lambert Law
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Beer-Lambert Law Limitations
Beer-Lambert Law Limitations
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Molar Absorption Coefficient
Molar Absorption Coefficient
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Ferroin
Ferroin
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Monochromatic Radiation Absorption
Monochromatic Radiation Absorption
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Path Length Effect
Path Length Effect
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Concentration Effect
Concentration Effect
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Intensity Change
Intensity Change
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Differential Form of Beer's Law
Differential Form of Beer's Law
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Integrated Beer's Law Equation
Integrated Beer's Law Equation
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Lambert-Beer's Law
Lambert-Beer's Law
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Absorbance (A) Formula
Absorbance (A) Formula
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Transmittance (T) Formula
Transmittance (T) Formula
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A from Percent Transmittance
A from Percent Transmittance
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T vs. Concentration
T vs. Concentration
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Beer-Lambert Law
Beer-Lambert Law
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Transmittance to %T
Transmittance to %T
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Transmittance to Absorbance
Transmittance to Absorbance
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Units of Molar Absorption
Units of Molar Absorption
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Study Notes
Absorption of Light
- When a sample doesn't absorb incident light, intensity remains the same: I = I₀
- When a sample absorbs incident light, intensity decreases: I ≠ I₀, I < I₀
- Intensity is defined as the square of the amplitude
- Transmitted intensity decreases relatively to incident intensity
- Wavelength remains constant during absorption
Lambert-Beer's Law / Beer's Law
- Defines the relationship between the absorbance of light through a substance and the concentration of the substance, as well as the path length of the light beam
- A = εcl, where A is absorbance, ε is the molar absorption coefficient, c is the concentration, and l is the path length
Definition of Lambert-Beer's Law
- Definition: When monochromatic radiation passes through a homogeneous solution of an absorbing substance, the decrease in intensity of radiation is directly proportional to the thickness of the absorbing medium and concentration of the solution
- The intensity of monochromatic radiations decreases exponentially with an increase in thickness and concentration of the homogeneous absorbing solution
Derivation of Lambert-Beer's / Beer's Law
- Factors affecting the transmitted light intensity are the thickness of the sample/path length and the concentration of the sample/analyte
Derivation Formulae
- dI/dx ∝ cl where:
- I₀ and I represent the intensity of incident and transmitted radiation, respectively
- x is the thickness
- dI/I = Kcdx where:
- K: proportionality constant
- The minus sign indicates a reduction in intensity upon absorption
- Integrating yields:
- Absorbance (A)= log₁₀ (I₀/I) where log₁₀ (I₀/I) = εcl
- A = εcl is the Lambert Beer's Law equation
- A = Absorbance, ε = Molar absorption coefficient, c = Concentration of the sample/analyte, l = Path-length of the solution
- Transmittance (T) = I/I₀ where I/I₀ = e⁻ᴷᶜˡ
Relationship Between Absorbance and Transmittance
- Transmittance (T) = I/I₀ and Absorbance (A)= log₁₀ (I₀/I)
- A = log₁₀ (1/T) = -log₁₀(T) and therefore T = 10⁻ᴬ = 10⁻ᵉᶜˡ
- If Transmittance(T) is expressed in percentage as %T = T x 100 yields T = %T/100
- A rewrite of the above is A = log₁₀(1/T) as: A = log₁₀(100/%T) = log₁₀(100) – log₁₀(%T)
- or, A = 2 – log₁₀(%T)
Solving Beer's Law Numericals
- Percentage of Absorbance (or Percent Absorption) is not applicable with the formula for Transmittance
- Conversion represents a double-headed arrow
- Formulae recap: T = I/I₀, A log₁₀(I₀/I)
- For %T = 100 x T
- To calculate the percentage of absorbance from absorbance: The path must be A → T → %T → %A'
- A = -log₁₀T and the inverse T = 10⁻ᴬ
- %A = 100 - %T
Units of Molar Absorption Coefficient
- A = εcl therefore ε = A/cl
- In convention:
- The unit of c is Molar (M) = mol/L = mol/dm³
- The unit / is a centimetre(cm)
- Absorbance is dimensionless
Properties of Molar Absorption Coefficient
- Units of ε are M⁻¹cm⁻¹ = Lmol⁻¹cm⁻¹ = dm³mol⁻¹cm⁻¹ = cm²mol⁻¹
- Constant
- Molecule-specific
- Wavelength-dependent
Use of Beer-Lambert Law
- Solves numerical problems
- Finds the Unknowns
Limitations of Lambert-Beer Law
- Only applicable to homogeneous and dilute solutions
- The radiation used should be monochromatic
- Solute must not undergo association, dissociation, polymerization, or hydrolysis in the solvent
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Description
Questions cover Beer-Lambert Law, absorbance, transmittance. Includes calculating absorbance, the effect of path length and concentration on absorbance, and the relationship between incident and transmitted light. These questions are designed to test understanding of spectrophotometry principles.
