Hi, sorry I missed out something. The titrant used was 0.100M Sodium thoiosulphate.Do I have to consider its concentration in the calculations?

Thanks a lot.

Posted by angela on Tuesday, August 3, 2010 at 6:58pm.


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.

Chemistry - please help - DrBob222, Tuesday, August 3, 2010 at 7:51pm
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 = ??

No, the molarity of the Na2S2O3 isn't needed since the mg/mL (titre) is given.

mL Na2S2O3 x mg/mL peroxide x 20 gives the mg peroxide directly.

To calculate the %w/w (weight by weight) of benzoyl peroxide in the sample, you need to follow the steps provided by DrBob222:

1. Calculate the amount of benzoyl peroxide in the 5 mL of solution used in the titration:
- Multiply the volume used in the titration (8.50 mL) by the conversion factor given: 8.50 mL x 12.11 mg/mL = ?? mg of benzoyl peroxide

2. Calculate the amount of benzoyl peroxide in the original 2.5011 g sample:
- Since the 5 mL solution was diluted from the original 2.5011 g sample, you need to multiply the amount of benzoyl peroxide in the 5 mL solution by the dilution factor.
- The dilution factor is calculated as follows: 100 mL (total volume) / 5 mL (volume used in the titration) = 20x
- Multiply the amount of benzoyl peroxide in the 5 mL by the dilution factor: ?? mg x 20 = ?? mg of benzoyl peroxide in the original 2.5011 g sample

3. Convert the amount of benzoyl peroxide in the sample from milligrams to grams:
- Divide the amount of benzoyl peroxide in the sample (in milligrams) by 1000 to convert it to grams.

4. Calculate the %w/w benzoyl peroxide in the sample:
- Divide the amount of benzoyl peroxide in grams by the initial sample weight (2.5011 g) and multiply by 100: %w/w = (grams peroxide / 2.5011 g) * 100