1

### AIPMT 2014

The pair of compounds that can exist together is
A
FeCl3, SnCl2
B
HgCl2, SnCl2
C
FeCl2, SnCl2
D
FeCl3, KI

## Explanation

Both FeCl2 and SnCl2 are reducing agents with low oxidation numbers.
2

### 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.
3

### 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.
4

### 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