Question

2mL of lycopene was extracted from 8 grams of tomato paste. The lycopene was analysed by UV-vis spectroscopy. The absorbance at 502nm was determined to be 0.162. The cell length was 1 cm. Determine the mass of the lycopene using the Beer-Lambert equation.

Answer 1

This problem is based on Beer-Lambert law. Mathematical equation is given as,

A = bC.........(1)

Where, A = Absorbance of the solution , b = path length of cuvtte (in cm unit), C concentration of sample.

= molar absorbance or Absroptivity (Extiction coefficient) = 1.72 x 10⁵ L.mol^-1.cm^-1. at 502 nm.

C is concentration in mol/L unit.

Molar mass of Lycopene = 536.873 g/mol.

Let find first concentration of lycopene in the unit mol/L in given solution.

We are given with, A = 0.162, b = 1 cm, (Extiction coefficient) = 1.72 x 10⁵ L.mol^-1.cm^-1. at 502 nm.

Put all these values in eq.(1)

Hence, 0.162 = 1.72 x 10⁵ x 1 x C

0.162 = 1.72 x 10⁵ x C

C = (0.162) / (1.72 x 10⁵)

C = 9.42 x 10^-7 mol/L

This is the concentration of Lycopene in 1L i.e. 1000 mL solution.

i.e. 1000 mL solution 9.42 x 10^-7 moles of Lycopene.

1000 mL solution [(9.48 x 10^-7 ) x Molar mass of Lycopene ] g .

1000 mL solution [(9.48 x 10^-7 ) x 536.873 g/mol ] g Lycopene

1000 mL solution   5.09 x 10^-4 g Lycopene.

SO, 2 mL solution (5.09 x 10^-4 ) / 500 g Lycopene...........(1000 mL / 500 = 2 mL)

2 mL solution 1.018 x 10^-6 g Lycopene.

= 1.018 g Lycopene. .......[ 10^-6 g = 1 g (microgram) ]

Hence mass of Lycopene is 1.018 microgram.