The answer depends on how much volume a droplet of water is. As it is, water drops can vary in sizes so we must define first the volume before we can perform the calculation.

The average volume of a single water drop is 0.05mL wherein there are 20 drops per milliliter. Deriving from the volume, there are over 1.5 sextillion molecules in a water drop and more than 5 sextillion atoms in it.

So how do we calculate the number of molecules and atoms in a droplet of water?

**Chemical Formula of Water**

- You have to determine first the chemical formula of water and this is
**H**. Meaning, there are two atoms of hydrogen and one atom of oxygen in a water molecule, summing_{2}0in one molecule of water.*3 atoms*

**Molar Mass of Water**

- Add up the mass of hydrogen atoms and oxygen atoms in a mole of water by searching the atomic mass of hydrogen and oxygen on a periodic table. The mass of hydrogen is 1.008 g/mol; whereas, the mass of oxygen is 16.00 g/mol. Let’s compute the molar mass of water using the given details:

Mass water = 2x mass hydrogen + mass oxygen

Mass water = 2 x 1.008 + 16.00

Mass water = 18.016 g/mol

- Thus, one mole of water has a mass of 18.016 grams.

**Water Density**

- Let us use the density of water to know the mass of water per unit volume. However, know that the density of water is flexible depending on its conditions. Warm water has less density but cold water has more density. Normally, the value we use in calculating is 1.00 g/mL. In other words, 1 millimiter of water has a mass of 1g. A drop of water is 0.05mL of water; hence the mass is 0.05g.

- One Mole of water is 18.016g; thus, in 0.05g the number of the mole is:

Moles of water = 0.05g x (1 mole/ 18.016g)

Moles of water = 0.002775 moles

**Avogrado’s Number: Knowing the Number of Molecules**

- Using the method of Avogradro’s number to determine the number of molecules in a drop of water. Avogrado’s number conveys that there are 60.022 x 1023 molecules of water per mole of water. Calculating the number of molecules in a drop of water, from which there are 0.002775 moles, we use the formula:

Molecules in a drop of water = (6.022 x 10^{23} molecules/mole) x 0.002275 moles

Molecules in a drop of water = 1.67 x 10^{21} water molecules

It is the same as saying that there is actually **1.67 sextillion water molecules within a water drop**.

Now, let us compute the number of atoms in a water drop:

Atoms in a water drop = 3 atoms/molecule x 1.67 x 10^{21} molecules

Atoms in a water drop = 5.01 x 10^{21} atoms

In other words, there are **5 sextillion atoms in a drop of water**.