Question: how many water molecules do you have?
Answer: Avogadro's number tells us there are 6.022 x 1023 molecules of water per mole of water. So, next we calculate how many molecules there are in a drop of water, which we determined contains 0.002775 moles: molecules in a drop of water = (6.022 x 1023 molecules/mole) x 0.002275 moles.