Question

Student measured the absorbance of substance X in aqueous solution depending on concentration. He used 0.5 cm cuvette for his measurements; concentration was expressed in moles/L and plotted on x-axis, while absorbance was on y-axis. His data were best fitted by the straight line with equation y = 2120x + 0.0112. The extinction coefficient for substance X in aqueous solution is:

Answer 1

Answer:

The extinction coefficient is 4240 M⁻¹cm⁻¹

Explanation:

The Beer-Lambert law for the absorbance of a monochromatic light by a molecule is given by the following mathematical expression:

$A=EbC$----------------------(1)

Here, E is the extinction coefficient, b is the length of light pathway through the solution and C is the concentration in mol/L. This is a linear equation and fits well with its formula  

y = mx + C-------------------(2)

Here, m is the slope and C is the y intercept. y and x are the two variables. Comparing  equation 1 and 2, Absorbance (A) and molar concentration (C) are the two variables, while the product of extinction coefficient (E) and path length (b) makes the slope of the line. The y-intercept in the given equation is due to the solvent interference.

From this information, we can determine the extinction coefficient by the slope of the line with the following formula.

$m=Eb$

$E=\frac{2120 (M^{-1} )}{0.5(cm)}$

$E= 4240 M^{-1}cm^{-1}$