6. The units of molar extinction coefficient are
   (1) L mol^-1 crn^-1 (2) cm^-1 mg ml^-1
   (3) mol^-1 mm (4) mol cm ml^-1

Correct Answer:

(1) L mol⁻¹ cm⁻¹

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Topic Explanation: Understanding the Units of Molar Extinction Coefficient (ε)

The molar extinction coefficient (ε), also known as the molar absorptivity, is a fundamental parameter in spectroscopy that relates how strongly a substance absorbs light at a particular wavelength.

It appears in the Beer-Lambert Law:
$A=\epsilon \times c\times l$

Where:

• $A$ is absorbance (unitless)

• $c$ is the concentration, expressed in moles per liter (mol L⁻¹)

• $l$ is the path length in centimeters (cm)

• $\epsilon$ is the molar extinction coefficient

To keep the absorbance unitless, the units of $\epsilon$ must balance those of concentration and path length:

• Concentration units: mol L⁻¹ (moles per liter)

• Path length units: cm

Thus, the units of $\epsilon$ become:
L mol⁻¹ cm⁻¹

This means the molar extinction coefficient expresses how much light (in absorbance) one mole of the substance absorbs in a 1-liter solution over a 1-cm path length at a specified wavelength (often 280 nm for proteins).

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Why Other Options Are Incorrect:

• (2) cm⁻¹ mg ml⁻¹: This set of units corresponds more to specific absorbance or mass extinction coefficients based on concentration in mg/ml, not molar units.

• (3) mol⁻¹ mm: Incorrect length unit (mm instead of cm), and incomplete dimensional consistency.

• (4) mol cm ml⁻¹: Incorrect dimension and inconsistent with Beer-Lambert law.

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Summary:

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Importance for CSIR NET, ICMR, DBT, IIT JAM & MSc Entrances

Understanding spectrophotometric parameters including the molar extinction coefficient and its units is crucial to answer questions on protein quantification, enzyme kinetics, and biochemical analytics. This foundational knowledge often appears in theoretical and applied questions in prestigious life science competitive exams.

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Answer: Option (1) L mol⁻¹ cm⁻¹ is the correct unit of the molar extinction coefficient.