How many molecules of water are in a single drop?
The simplest way is to know two things about the object:
1) Its chemical composition.
2) Its mass.
As an example, let’s say we have a very tiny drop of water, say 1 microliter, which would have a mass of 0.001 grams. The chemical formula for water is H2O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). From the Periodic Table of Elements, one sees that one mole of hydrogen atoms weighs 1 gram while one mole of oxygen atoms weighs 16 grams.
A mole is a convenient counting unit whenever one is dealing with numbers of atoms or molecules. It is equal to Avogadro’s number (NA), namely 6.022 x1023. If we have one mole of water, then we know that it will have a mass of 2 grams (for 2 moles of H atoms) + 16 grams (for one mole O atom) = 18 grams. Since we only have 0.001 grams, that means that we have NA * (0.001/18) = 3.35*1019 molecules of water! This is a huge number, even though we only started with a tiny amount of water.
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