Water molecule composed of two Hydrogen atoms and one Oxygen atom. Its molecular formula is H2O.
Experimental verification
   Composition of water can be determined by using an apparatus called "Hofmann voltameter" .
Construction of Hofmann voltameter
   The voltameter consists of two glass tubes of equal volume.
   Each glass tube is fitted with a platinum plate .
   One platinum plate is connected to positive terminal of power supply and it is called "Anode".
   The other plate is connected to negative terminal of power supply and it is called "Cathode".
   Glass tubes of voltameter contain pure water.
   When an electric current is passed through water containing a little salt, water decomposes into    hydrogen gas and oxygen gas. Hydrogen is collected at cathode. Oxygen is collected at anode.
   The tube on left side contains two volumes of H2 while the other tube has one volume of O2 .
   This experiment shows that :
Composition of water by Volume
   Water composed of two volumes of hydrogen and one volume of oxygen.
Composition of water by parts
   Hydrogen =1 part by mass.
   Oxygen = 8 parts by mass.
Composition of water in percentage Composition
   Hydrogen =11.11 %
   Oxygen = 88.89 %
Molecular Formula of Water
   In the light of above facts it is concluded that the molecular formula of water is H2O.