# Volume strength of H2O2 using iodometric titration

Gold Member

## Homework Statement

To a 25ml H2O2 solution, excess acidified solution of potassium iodide is added. The libereated iodine required 20ml of 0.3N sodium thiosulphate solution. Calculate the volume strength of H2O2.

## Homework Equations

Normality = n-factor × Molarity
Volume strength = Molarity × 11.2

## The Attempt at a Solution

I will use moles and not equivalence. But to do that, I need to convert normality to molarity.

For the reaction of sodium thiosulphate with iodine, n-factor is 2 for sodium thiosulphate.
Hence M = N/n = 0.15.
So number of milli moles of thiosulphate is 3.

$$I_2 + 2Na_2S_2O_3 \rightarrow 2Na_2S_4O_6 + 2NaI$$

So number of millimoles of iodine is ##\frac{3}{2}##.

$$2H^+ + H_2O_2 + 2KI \rightarrow 2H_2O+I_2+2K^+$$

Hence millimoles of hydrogen peroxide is ##\frac{3}{2}##

So molarity of hydrogen peroxide is ##\frac{3}{50}##

So volume strength = ##\text{M}\times 11.2 = 0.672##

But the answer given is double of what I got.

$$2e^-+2S_2O_3^{2-}\rightarrow S_4O_6^{2-}$$