Volume strength of H2O2 using iodometric titration

  • #1
Titan97
Gold Member
451
18

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.
 

Answers and Replies

  • #2
Titan97
Gold Member
451
18
oops...
The number of electrons exchanged by 1 mole of thiosulphate is 1.
$$2e^-+2S_2O_3^{2-}\rightarrow S_4O_6^{2-}$$

Hence n-factor is 1 and not 2. Now I got the given answer.
 

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