Question

Assume the molar absorptivity (ε) for CoCl42- at 700 nm is 409.7 M-1 cm-1. If d, the cell path length, is equal to 1.48 cm for the cuvettes being used, and if the measured absorbance (A) at 700 nm is 1.147, what is [CoCl42-]?

Answer 1

Answer:

1.89 × 10⁻³ M

Explanation:

In a UV Visible Spectrometer, we can determine the concentration of a species using Beer-Lambert law.

A = ε × l × c

where,

• A: absorbance

• ε: molar absorptivity

• l: the cell path length

• c: concentration of the species

We can find  [CoCl₄²⁻] from this expression.

A = ε × l × c

c = A / ε × l

c = 1.147 / 409.7 M⁻¹ cm⁻¹ × 1.48 cm

c = 1.89 × 10⁻³ M

Answer 2

Molar absorptivity gives the absorption power of the chemical species at a wavelength. The concentration of $\rm [CoCl_{4}^{2-}]$ species is $1.89 \times 10^{-3}\;\rm M.$

What is Beer-Lambert law?

Beer-Lambert law is the relation of the absorbance of the species with the molar absorptivity, concentration of the species present and the length of the light path travelled.

It can be given as:

$\rm A = \epsilon \times l \times c$

Given,

Absorbance (A) = 1.147

Molar absorptivity (ε) = 409.7 per M per cm

Cell path length (l) = 1.48 cm

Substituting values in the above equation concentration can be calculated as:

$\begin{aligned}\rm c &=\rm \dfrac{A}{ \epsilon \times l}\\\\&= \dfrac{1.147}{409.7\times 1.48}\\\\&= 1.89 \times 10^{-3} \;\rm M\end{aligned}$

Therefore, the concentration of the species is $1.89 \times 10^{-3}.$

Learn more about Beer-Lambert law here:

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