How to calculate amount of substance in phycocyanin tablet? please help
a. 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?
Molar mass of phycocyanin=586.69
a. 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?
Molar mass of phycocyanin=586.69
1 Answer
I got
Since the first goal is to get the
Recall Beer's law:
A = epsilonbc where:
A is the absorbance.epsilon = "229 L/mol"cdot"cm" is the molar absorptivity (extinction coefficient) of phycocyanin typically in"L/mol"cdot"cm" .b = "1 cm" is the path length of a typical cuvette.c is the concentration of the sample in typically"mol/L" .
Let's track what has been going on.
- Five tablets were prepared,
"1.2 g" in"100 mL" of ethanol. The initial concentration is"12 g tablet/L" (and some particles were filtered out later that did not dissolve past"12 g/L" ). - When
"50 mL" of the solution was used, the concentration was unaltered from the"100 mL" solution. "2 mL" of the solution was diluted 10-fold, so you now have"1.2 g/L" of the tablet in solution. The tablet has a certain mass percentage of phycocyanin.- This particular sample of unknown concentration has an absorbance of
0.324 , of only the phycocyanin.
Note that
So, we will need to find the concentration using the absorbance, not through the dilution logic.
c = A/(epsilonb) = 0.324/("229 L/mol"cdotcancel"cm" cdot cancel"1 cm")
= "0.00141 mol/L phycocyanin"
Or, using its molar mass would give it in
(0.00141 cancel"mol")/"L" xx "586.69 g phycocyanin"/cancel"mol"
= "0.830 g/L"
Now, the key point is that it was of the
"0.830 g phycocyanin"/(1000 cancel"mL") xx 20 cancel"mL" = "0.0166 g phycocyanin"
in
[At this point we COULD determine the concentration of the phycocyanin in the starting solution already, but I think the following argument is more useful.]
Since mass and volume are extensive, and concentration is intensive we can say that
"0.0166 g phycocyanin"/"2 mL solution" = "0.415 g phycocyanin"/"50 mL solution"
after filtering undissolved particles out. Before filtering undissolved particles out,
"0.415 g phycocyanin"/"50 mL solution" = color(green)("0.830 g phycocyanin"/"100 mL solution") which, by the way, is also
"8.30 g phycocyanin"/"L solution" .
That's what we expected from back-calculating from a 10-fold dilution of the
Now that we know how much of the
We can say that before grinding the tablets,
"0.830 g phycocyanin"/"1.2 g crushed tablets" = "1.73 g phycocyanin"/"2.5 g whole tablets"
or
"1730 mg phycocyanin"/"2500 mg whole tablets"
or
color(blue)("346 mg phycocyanin"/"500 mg tablet")
Therefore, there are
= color(blue)ul(69%"w/w")
to two sig figs.
