A stock solution's hypochlorite activity was determined as follows: . -
2.00g of liquid sodium hypochlorite (NaOCl) was weighed into a flask and diluted with 50mL deionized water.10mL of 10% sulfuric acid and 10mL of 10% potasssium iodide were added, followed by titration with 0.1N sodium thiosulfate (Na2S2O3) until colorless.
% sodium hypochlorite
= (mL Na2S2O3)(0.1N)(0.03722)/2.00gNaOCl X 100
[The factor 0.03722 was derived by multiplying molecular weight of NaOCl (74.45g/mol) by 0.001 (mL to L conversion) and dividing by 2(mols of thiosulfate per hypochlorite).]
Please explain why we need to multiply by 0.1N? Thanks.
Break it down into steps.
Step 1 is to calculate #mew.
mL x N = number of milliequivalents.
Step 2 convert #mew to grams.
number of milliequivalents x milliequivalent weight = grams.
Step 3 converts grams to percent
(grams/weight sample) x 100 = percent.
The reason we need to multiply by 0.1N is because it represents the normality of the sodium thiosulfate (Na2S2O3) solution that is used for titration.
Normality is a unit of concentration that measures the number of equivalents of a substance per liter of solution. In this case, the 0.1N sodium thiosulfate solution means that there is 0.1 equivalent of sodium thiosulfate per liter of solution.
During the titration, the sodium thiosulfate reacts with the iodine that is formed by the reaction between the potassium iodide and the hypochlorite. The reaction is as follows:
2Na2S2O3 + I2 → Na2S4O6 + 2NaI
In this reaction, each mole of sodium thiosulfate reacts with one mole of iodine. Since the molecular weight of Na2S2O3 is 158.11 g/mol, each mole of Na2S2O3 corresponds to 158.11 grams.
Now, in the equation to calculate the percentage of sodium hypochlorite, we have:
% sodium hypochlorite = (mL Na2S2O3)(0.1N)(0.03722)/2.00gNaOCl X 100
Here, the mL Na2S2O3 represents the volume of the sodium thiosulfate solution used in the titration. By multiplying this volume by 0.1N, we are converting the volume to the number of equivalents of sodium thiosulfate reacted.
The factor 0.03722 is derived from the molecular weight of NaOCl (74.45g/mol) multiplied by 0.001 (to convert mL to L) and divided by 2 (mols of thiosulfate per hypochlorite). This factor is used to convert the amount of sodium thiosulfate used in the reaction to the amount of sodium hypochlorite present in the stock solution.
So, multiplying by 0.1N is necessary to account for the concentration of the sodium thiosulfate solution used in the titration and to convert the volume of sodium thiosulfate reacted to the number of equivalents of sodium thiosulfate.