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Introduction to Oxidation and Reduction

The indicator...

Question

The indicator used in the titration of iodine against sodium thiosulphate is:

A

Starch

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B

K_{3} F e (C N)_{6}

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C

K_{2} C r O_{4}

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D

Potassium

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Solution

The correct option is A Starch
In an iodometric titration, a starch solution is used as an indicator since it can absorb the $I_{2}$ that is released. This absorption will cause the solution to change its colour from deep blue to light yellow when titrated with a standardized thiosulfate solution. This indicates the endpoint of the titration.

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0.1 g K I O_{3}

K I

when treated with

H C l

, the iodine is liberated. The liberated iodine required

45 m L

sodium thiosulphate for titration. The molarity of sodium thiosulphate will be equivalent to:

25 m L H_{2} O_{2}

were added to excess of acidified solution of

K I

. The iodine so liberated required

20 m L

0.1 N

sodium thiosulphate for titration. Calculate the strength in terms of normality percentage and volume.

Q. In iodometric titrations, an oxidizing agent such as

K M n O_{4}, K_{2} C r_{2} O_{7}, C u S O_{4}, H_{2} O_{2}

is allowed to react in neutral medium or in acidic medium with excess of potassium iodide to liberate free iodine

(I_{2})

.
Free iodine is titrated against standard reducing agent usually with sodium thiosulphate. The reactions are as follows:

K_{2} C r_{2} O_{7} + 6 K I + 7 H_{2} S O_{4} \to C r_{2} (S O_{4})_{3} + 4 K_{2} S O_{4} + 7 H_{2} O + I_{2}

2 C u C O_{4} + 4 K I \to C u_{2} I_{2} + 2 K_{2} S O_{4} + I_{2}

I_{2} + N a_{2} S_{2} O_{3} \to 2 N a I + N a_{2} S_{4} O_{6}

In iodometric titrations, starch solution is used as an indicator. Starch solution gives blue or violet colour with free iodine. At the end point, blue or voilet colour disappears when iodine is completely changed to iodide.

What volume of 0.40 M

N a_{2} S_{2} O_{3}

would be required (in mL) to react with

I_{2}

liberated by adding 0.04 mole of

K I

to 50 mL of 0.20 M

C u S O_{4}

0.5 g

K_{2} C r_{2} O_{7}

K M n O_{4}

was treated with an excess of

K I

in acidic medium. Iodine liberated required

100 c m^{3}

0.15 N

sodium thiosulphate solution for titration. Find the percentage amount of each in the mixture.

Q. In the titration of Mohr salt against

K M n O_{4}

the indicator used is:

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Introduction to Oxidation and Reduction

Standard XII Chemistry

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