Please help to determine the (%w/w) benzoyl peroxide (C14H20O4) in a sample.
Procedure: 2.500g of sample was dissolved in 75ml dimethylformamide and diluted to 100.0ml with the same solvent. (Solution T). To 5.0ml of Solution T, 20ml of acetone and 3ml of a 500g/l solution of potassium iodide were added and mixed .This was allowed to stand for 1min and then titrated with 0.1M sodium thiosulphate using 1ml of starch solution added towards the end of the titration as indicator. A blank titration was carried out.
1ml of 0.1M sodium thiosulphate is equivalent to 12.11mg of C14H20O4.
The following data were obtained based on 2 titre values: -
Sample Weight 1 = 2.5011g, titrant volume = 8.58ml
Sample Weight 2 = 2.5030g, titrant volume = 8.58ml
Blank titrant volume= 0.80ml
How do I calculate (%w/w) benzoyl peroxide in the sample?
Thanks.
8.58 mL - 0.80 blank = 8.50 mL used in the titration.
8.50 mL x 12.11 mg/mL = ??mg of the stuff in the 5 mL. That x 20 (100 mL/5 mL = 20x) = ?? mg stuff in the original 2.5011 g sample.
Change that (the mg peroxide) to grams, and convert to percent.
Percent = grams peroxide/2.5011)*100 = ??
To calculate the (%w/w) benzoyl peroxide in the sample, you need to follow the steps below:
1. Calculate the volume of titrant used for the sample:
Titrant volume = Titrant volume (sample) - Titrant volume (blank)
= 8.58 ml - 0.80 ml
= 7.78 ml
2. Convert the volume of titrant used to moles of sodium thiosulphate:
Moles of sodium thiosulphate = Titrant volume (in L) x Molarity of sodium thiosulphate
= 7.78 ml x 0.1 mol/L
= 0.778 mmol
3. Calculate the moles of benzoyl peroxide in 5.0 ml of Solution T based on the stoichiometry of the reaction:
According to the given information, 1 ml of 0.1M sodium thiosulphate is equivalent to 12.11 mg of C14H20O4.
So, 0.778 mmol of sodium thiosulphate is equivalent to:
0.778 mmol x (12.11 mg / 1 ml) = X mmol x (mg C14H20O4 / 5 ml)
Solve for X (moles of benzoyl peroxide):
X mmol = 0.778 mmol x (12.11 mg / 1 ml) / (mg C14H20O4 / 5 ml)
X mmol = 0.778 mmol x (12.11 mg x 5 ml) / (1 ml x mg C14H20O4)
X mmol = 47.0505 mmol
4. Calculate the weight of benzoyl peroxide in the sample:
Weight of benzoyl peroxide (in mg) = X mmol x (mg C14H20O4 / 1 mmol)
Weight of benzoyl peroxide (in mg) = 47.0505 mmol x (mg C14H20O4 / 1 mmol)
5. Calculate the (%w/w) benzoyl peroxide in the sample:
(%w/w) benzoyl peroxide = Weight of benzoyl peroxide (in mg) / Sample weight (in g) x 100
(%w/w) benzoyl peroxide = Weight of benzoyl peroxide (in mg) / (2.5011 g or 2.5030 g) x 100
Substitute the appropriate values into the equations mentioned above to calculate the (%w/w) benzoyl peroxide in the sample.