1

### 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.
2

### 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
3

### 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-
4

### AIPMT 2004

Which is the best description of the behaviour of bromine in the reaction given below?
H2O + Br2 $\to$ HOBr + HBr
A
Proton acceptor only
B
Both oxidised and reduced
C
Oxidised only
D
Reduced only

## Explanation

H2O + ${\mathop {Br}\limits^0}$2 $\to$ HO$\mathop {Br}\limits^{ + 1}$ + H$\mathop {Br}\limits^{ - 1}$

In the above reaction the oxidation number of Br2 increases from zero (in Br2 ) to +1 (in HOBr) and decreases from zero (in Br2 ) to –1 (in HBr). Thus Br2 is oxidised as well as reduced and hence it is a redox reaction.