# Iodide Oxidation by Hydrogen Peroxide

In my chemistry lab we prepared the following solutions:
Buffered Iodide Solution
25 mL acetic acid/sodium acetate buffer
6mL of 0.5 % starch solution
6 mL of 1.0M KI
fill to the 250 mL mark with distilled water

0.025M Thiosulfate Solution
5 ml of 1 M Na2S2O3
fill to the 200 ml mark with distilled water

Peroxide Soution
30ml of 1 M H2O2
fill to the 150 ml mark with distilled water

Then mixing these solutions together:
Mixture 1:
Buffered Iodide 40ml
Thiosulfate Solution 10ml
Distilled Water 10 ml

Mixture 2:
Buffered Iodide 40ml
Thiosulfate Solution 10ml
Distilled Water 20ml

Mixture3:
Buffered Iodide 40ml
Thiosulfate Solution 10ml

Mixture 4:
Buffered Iodide 40ml
Thiosulfate Solution 5 ml
Distilled Water 5ml

Separately put each beaker into ice water and measure the following amounts of peroxide solution separately into the container of ice:
Mixture 1: 20ml
Mixture 2: 10ml
Mixture 3: 30ml
Mixture 4: 30ml
when the two flasks have cooled on ice below 3°C add the peroxide

I am unsure for my lab questions what values to use for each trial to calculate the moles of sodium thiosulfate reacted? and how to calculate the mol/L H2O2 reacted

chemisttree
Homework Helper
Gold Member
Strangest lab I've ever seen.

Borek
Mentor
0.025M Thiosulfate Solution
5 ml of 1 M Na2S2O3
fill to the 200 ml mark with distilled water
How many moles of thiosulfate in 5 mL of 1M solution?

What is the concentration of solution that contains this number of moles of thiosulfate in 200 mL?

wouldn't the concentration be 1 M

Borek
Mentor
No. Don't guess, try to calculate, following exactly questions as I listed them. This is mostly a simple plug and chug - use the definition of the concentration rearranged to isolate thing that you want to calculate.