Question

4.
A solution of chemical ‘A’ having its 0.14 mol L concentration has an absorbance of 0.42. Another
solution of 'A' under the same conditions has an absorbance of 0.36. What is the concentration of this
solution of 'A'?
a) 0.808 mol L-
b) 0.35 mol L'
c) 0.12 mol L'
d) 0.10 mol L-!
​

Answer 1

Given,

For a solution of chemical ‘A’;

Case-I

When the molarity of the solution is 0.14 mol L, absorbance = 0.42

Case-II

The absorbance of the solution = 0.36

To find,

The molarity of the solution when its absorbance is 0.36.

Solution,

We can simply solve this numerical problem by using the following process:

Theoretically,

The standard equation for absorbance is represented as;

A = ɛ x l x c, {Equation-1}

where A = the amount of light absorbed by the sample for a given wavelength

ɛ = the molar absorptivity

l = the distance that the light travels through the solution

c = the concentration of the absorbing species

Now,

for the given solution of chemical A, the value of ɛ and l remains unchanged irrespective of the change in the concentration and absorbance of the solution.

So, from equation-1, we get;

A = ɛ x l x c

=> ɛ x l = A/c = constant for the given solution of chemical A {Equation-2}

Now, according to the question;

A/c ratio in case-I = A/c ratio in case-II

=> 0.42/0.14 = 0.36/c

=> c = (0.36 x 0.14) / 0.42

=> c = 0.12 mol/L

Hence, the molarity of the solution is equal to 0.12 mol/L when its absorbance is 0.36.