1

### 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-
2

### 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.
3

### AIPMT 2003

The oxidation states of sulphur in the anions

SO3${_3^{2 - }}$, S2O${_4^{2 - }}$ and S2O${_6^{2 - }}$ follow the order
A
S2O${_4^{2 - }}$ < SO${_3^{2 - }}$ < S2O${_6^{2 - }}$
B
SO${_3^{2 - }}$ < S2O${_4^{2 - }}$ < S2O${_6^{2 - }}$
C
S2O${_4^{2 - }}$ < S2O${_6^{2 - }}$ < SO${_3^{2 - }}$
D
S2O${_6^{2 - }}$ < S2O${_4^{2 - }}$ < SO${_3^{2 - }}$

## Explanation

SO32– : oxidation state of ‘S’ is +4

S2O42– : oxidation state of ‘S’ is +3.

S2O62– : oxidation state of ‘S’ is +5.

So, the order is S2O${_4^{2 - }}$ < SO${_3^{2 - }}$ < S2O${_6^{2 - }}$.