Question

The number of water molecules present in a drop of water (volume 0.0018 ml) at room temperature is

Answer 1

Density of water is  1 gm / ml
So mass of drop of water given is  (0.0018 ml  )  => 0.0018 gm

In 18 grams of water , we have 1 mole of molecules - Avogadro number.  So
6.0225 * 10^23 molecules.

so number of molecules in the given drop of water =
       0.0018 * 6.0225 * 10^23 = 1.08425 * 10^21 molecules