HELP ASAP please

Question

The extinction coefficient is 250 and the molar mass is about 30000 ( that is what I found online) You want to determine the amount of Phycocyanin in a 500 mg herbal tablet. Before analysis by UV-Vis spectroscopy, five Phycocyanin tablets (each 500 mg) were crushed in a pestle and mortar. 1.2 g of the crushed tablets were then dissolved in 100 ml of ethanol. 50 ml of the solution was filtered to remove undissolved particles. 2 ml of the filtered solution was then diluted by a factor of 10 (i.e., dilution factor is 10). The diluted sample was analysed using UV-Vis spectroscopy and had an absorbance of 0.324. Show each calculation step clearly. What was the amount in mg of Phycocyanin in a 500 mg tablet? What is the percent (%w/w) of Phycocyanin in a tablet?

Answer 1

Ok. But the 250 is NOT the extinction coefficient.  I found it to be 2.3x10⁵ M^-1cm^-1 which makes more sense. So here is the approach.

A = ecl

0.324 = 2.3x10⁵ x c x 1

c = 0.324/2.3x10⁵

c = 1.41x10^-6 M = 1.41x10^-6 moles/liter

1.41x10-6 moles/liter x 30,000 g/mole = 4.23x10^-2 g/liter = 42.3 mg/liter

This is the concentration in the 2 ml filtrate that was diluted 10x

Concentration of phycocyanin in the original filtrate = 10 x 42.3 ml/liter = 423 mg/liter

Since you had 50 ml (0.05 liters) and not 1 liter, then you had 423 mg/liter x 0.05 liter = 21.15 mg in the 50 ml

This was taken out of 100 ml so in the 100 ml you had 2 x 21.15 mg = 42.3 mg of phycocyanin.

This was equivalent to 1.2 g of tablet so you have 42.3 mg/1.2 g tablet = 35.25 mg/g tablet

500 mg is 0.5 g, so 35.25 mg/g x 0.5 g = 17.625 mg phycocyanin/500 mg tablet

To two significant figures this is 18 mg phycocyanin/500 mg tablet

%(w/w) = 18 mg/500 (x100%) = 3.6% (w/w/)