Question: How many mollecules of water are there in one drop?

James M Monk answered on 13 Jun 2011:

Let us guess that a drop of water has a volume of about a tenth of a millilitre, which has a mass of 0.1 grams. Water has an atomic mass of 18, so one mole of water has a mass of 18 grams. 0.1 grams of water is therefore approximately 1 / 180th of a mole. One mole contains 6 x 10^23 molecules, therefore a drop of water contains approximately (6 / 180) * 10^23 = 3 * 10^21 molecules of water. Since I guessed the volume of a drop to be 0.1 millilitres, when it could be more or less than that, I will give a final answer of somewhere between 10^21 and 10^22 molecules