How many molecules are contained in one drop (0.050 mL) of water (density 1.0 g/mL)?
To answer this question one must first start with the concept of a mole.
The mole is a number 6.022 x10^23 that can represent anything. This number is known as Avogadro's number. One would like to own a mole of pennies for instance.
The mole is only convenient when speaking ...
How many water molecules are there in a drop? This is a concept that is poorly understood, difficult to visualize, but relatively easy to calculate.