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1

### AIPMT 2012 Prelims

A mixture of potassium chlorate, oxalic acid and sulphuric acid is heated. During the reaction which element undergoes maximum change in the oxidation number?
A
S
B
H
C
Cl
D
C

## Explanation

KClO3 + C2H2O4 + H2SO4 $$\to$$ K2SO4 + CO2 + KCl + H2O

Maximum change in oxidation number of chlorine, i.e., from +5 to –1.
2

### AIPMT 2012 Prelims

When Cl2 gas reacts with hot and concentrated sodium hydroxide solution, the oxidation number of chlorine changes from
A
zero to +1 and zero to $$-$$5
B
zero to $$-$$1 and zero to +5
C
zero to $$-$$1 and zero to +3
D
zero to +1 and zero to $$-$$3

## Explanation

This is an example of disproportionation reaction and oxidation state of chlorine changes from 0 to –1 and +5.
3

### AIPMT 2009

Oxidation numbers of P in PO$$_4^{3 - }$$, of S in SO$$_4^{2 - }$$ and that of Cr in Cr2O$$_7^{2 - }$$ are respectively
A
+3, +6 and +5
B
+5, +3 and +6
C
$$-$$3, +6 and + 6
D
+5, +6 and +6

## Explanation

Let oxidation number of P in PO4 3– be x.

$$\therefore$$ x + 4(–2) = –3

$$\Rightarrow$$ x = +5

Let oxidation number of S in SO4 2– be y.

$$\therefore$$ y + 4(–2) = –2

$$\Rightarrow$$ y = +6

Let oxidation number of Cr in Cr2O7 2– be z.

$$\Rightarrow$$ 2z + 7(–2) = –2

$$\Rightarrow$$ z = +6
4

### AIPMT 2008

Number of moles of MnO$$_4^{ - }$$ required to oxidize one mole of ferrous oxalate completely in acidic medium will be
A
7.5 moles
B
0.2 moles
C
0.6 moles
D
0.4 moles

## Explanation

The balanced ionic equation for oxidation of ferrous oxalate by MnO4 in acidic medium is as follows :

3MnO4- + 5FeC2O4 + 24H+ $$\to$$

3Mn2+ + 10CO2 + 12H2O + 5Fe3+

Thus, 5 moles of FeC2O4 require 3 moles of MnO4-.

So, 1 mole of FeC2O4 requires

= $${3 \over 5}$$ = 0.6 moles of MnO4-

#### Questions Asked from Redox Reactions

On those following papers in MCQ (Single Correct Answer)
Number in Brackets after Paper Indicates No. of Questions
NEET 2020 Phase 1 (1)
NEET 2019 (1)
NEET 2018 (3)
NEET 2016 Phase 2 (1)
AIPMT 2014 (3)
AIPMT 2012 Prelims (2)
AIPMT 2009 (1)
AIPMT 2008 (1)
AIPMT 2004 (1)
AIPMT 2003 (1)

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Class 12