Question

A) 7mL of a 0.1M CaCl2 solution was diluted to a final volume of 150mL. Please determine the concentration of Cl- in this solution.

B) Explain why creating a standard curve is better than simply using the Beer-Lambert law directly. Assume that a value for molar absorptivity is readily available.

Answer 1

A)

Initial Volume of the solution =

Initial concentration of the solution

The above solution is diluted to a final volume of

Hence, the final concentration can be calculated by using the conservation of number of moles of substance in dilution.

Now, each molecule of CaCl2 dissociates as follows

Hence, 1 mol of CaCl2 produces 2 mol of Cl- ions.

Hence, the concentration of Cl- ions will be twice that of CaCl2. Hence, the concentration of Cl- in the diluted solution is

Hence, the answer is 0.0093 M

B)

The Beer-Lambert's Law relates the absorbance of a solution to its concentration as follows

Where A = absorbance

C = concentration

l = path length

= molar absorptivity

Now, we usually use the absorbance values to determine a unknown concentration using a standard curve fitting from standardisation data instead of directly using the equation above. It is because sometimes we use this to determine the unknown in a mixture of solutions with more than two components. Hence, we can not have an absorbance of zero when the concentration of one of the unknown component is actually zero, hence it would obviously not obey the above equation.

But in the standardised equation

Hence, by plotting Absorbance vs concentration we get an intercept of which accurately says that the absorbance of the solution may not be zero at zero concentration of our unknown. It is very much essential to determination of unknown components in a mixture of components.