State two conditions necessary to write the molecular formula of a compound.

State the number of atoms of each element, present in

(a) C₆H₁₂O₆

(b) H₂SO₄

(c) HNO₃

(d) CaCO₃

Also name these compounds.

Mention three gaseous elements and write their molecular formulae.

State the information obtained from the formula of a compound.