1

### AIPMT 2015

20.0 g of a magnesium carbonate sample decomposes on heating to give carbon dioxide and 8.0 g magnesium oxide. What will be the percentage purity of magnesium carbonate in the sample?
(At. wt. of Mg = 24)
A
96
B
60
C
84
D
75

## Explanation

MgCO3 $\buildrel \Delta \over \longrightarrow$ MgO + CO2

84 g of MgCO3 $\equiv$ 40 g of MgO

$\therefore$ 20 g of MgCO3 = ${{40} \over {84}} \times 20$ = 9.52 g of MgO

But Actual yield = 8 g of MgO

$\therefore$ % purity = ${8 \over {9.52}} \times 100$ = 84%
2

### AIPMT 2015

On heating which of the following releases CO2 most easily?
A
Na2CO3
B
MgCO3
C
CaCO3
D
K2CO3

## Explanation

Carbonates becomes more thermally stable down the group, Also, the alkali metal carbonate are more stable than alkaline earth metal carbonates. Therefore MgCO3 will leave CO2 easily.

MgCO3 $\buildrel \Delta \over \longrightarrow$ MgO + CO2
3

### NEET 2013 (Karnataka)

In Castner-Kellner cell for production of sodium hydroxide
A
brine is electrolyzed using graphite electrodes
B
molten sodium chloride is electrolysed
C
sodium amalgam is formed at mercury cathode
D
brine is electrolyzed with Pt electrodes

## Explanation

In Castner-Kellner cell, sodium amalgam is formed at mercury cathode.
4

### AIPMT 2012 Prelims

The ease of adsorption of the hydrated alkali metal ions on an ion-exchange resins follows the order :
A
Li+ < K+ < Na+ < Rb+
B
Rb+ < K+ < Na+ < Li+
C
K+ < Na+ < Rb+ < Li+
D
Na+ < Li+ < K+ < Rb+

## Explanation

All alkali metal salts are ionic (except Lithium) and soluble in water due to the fact that cations get hydrated by water molecules. The degree of hydration depends upon the size of the cation. Smaller the size of a cation, greater is its hydration energy.

Li+ < K+ < Na+ < Rb+

Relative ionic radii in water or relative degree of hydration:

Rb+ < K+ < Na+ < Li+