1

AIPMT 2008

The sequence of ionic mobility in aqueous solution is
A
Rb+ > K+ > Cs+ > Na+
B
Na+ > K+ > Rb+ > Cs+
C
K+ > Na+ > Rb+ > Cs+
D
Cs+ > Rb+ > K+ > Na+

Explanation

Smaller the size of an ion, greater will be the degree of hydration. So, degree of hydration is highest for Li+ and that is smallest for Cs+. Therefore, Li+ holds the water molecules in its hydration sphere and Cs+ ion holds the least water molecules.

Hence, the correct ionic mobility in aqueous medium is :

Cs+ > Rb+ > K+ > Na+
2

AIPMT 2008

The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following orders?
A
NaH > LiH > KH > RbH > CsH
B
LiH > NaH > KH > RbH > CsH
C
CsH > RbH > KH > NaH > LiH
D
KH > NaH > LiH > CsH > RbH

Explanation

The stability of alkali metal hydrides decreases from Li to Cs. It is due to the fact that M–H bonds becomes weaker with increase in size of alkali metals as we move down the group from Li to Cs. Thus the order of stability of hydrides is

LiH > NaH > KH > RbH > CsH
3

AIPMT 2008

Equimolar solutions of the following were prepared in water separately. Which one of the solutions will record the highest pH?
A
MgCl2
B
CaCl2
C
SrCl2
D
BaCl2

Explanation

Equimolar solutions of the given chlorides when prepared in water forms their respective hydroxides.

SrCl2 + 2H2O $\to$ Sr(OH)2 + 2HCl

BaCl2 + 2H2O $\to$ Ba(OH)2 + 2HCl

MgCl2 + 2H2O $\to$ Mg(OH)2 + 2HCl

CaCl2 + 2H2O $\to$ Ca(OH)2 + 2HCl

Be(OH)2 is amphoteric, but the hydroxides of other alkaline earth metals are basic. The basic strength increases down the group. Hence higher the basic character higher will be the pH.
4

AIPMT 2007

In which of the following the hydration energy is higher than the lattice energy?
A
MgSO4
B
RaSO4
C
SrSO4
D
BaSO4

Explanation

Hydration energy of sulphate decreases on moving down the group-II. Mg2+ is smaller than other ions of group II, so Mg2+ is readily hydrated and hence MgSO4 has higher hydration energy than the lattice energy.