(i) Write the molecular formula of magnesium phosphate. 
(ii) Calculate the molecular mass of this compound. 
(iii) What is the percentage of phosphorus in magnesium phosphate?  [At. mass: $\mathrm{Mg} = 24, \mathrm{P} = 31, \mathrm{O}=16$]

Give a reason for the following:

The physical properties of isotopes are different.

Give reason for the following:

Concentrated sulphuric acid is used as a dehydrating agent.

Give reason for the following:

${\mathrm{AlCl}}_3$ is known as aluminium chloride while ${\mathrm{PCl}}_3$ is known as phosphorous trichloride.

Give reason for the following:

• Hydrogen is kept in I A group.

Give reason for the following:

• Table salt absorbs moisture during the rainy season.

Name the type of chemical reaction for the following chemical equation:

$2{\mathrm{SO}}_2+{\mathrm{O}}_2\stackrel{{\mathrm{V}}_2{\mathrm{O}}_5}{\rightleftharpoons }2{\mathrm{SO}}_3$

Name the type of chemical reaction for the following chemical equation:

${\mathrm{CuSO}}_4+\mathrm{Fe}\to {\mathrm{FeSO}}_4+\mathrm{Cu}$

Name the type of chemical reaction for the following chemical equation:

$\mathrm{PbO}+{\mathrm{SO}}_2\to {\mathrm{PbSO}}_3$