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.
