# Number of Atoms and Molecules Present in one water Drop.

Did you ever think how many atoms are in a drop of water or how many molecules are in a single water drop? The answer will depend on your definition of the volume of a water droplet. The average volume of a water drop is precisely 0.05 mL (20 drops per millilitre). It turns out that there are over 1.5 sextillions (1^21) molecules in a water drop and more than five sextillion atoms per water droplet.

**Water Chemical Formula.**

To calculate the number of atoms and molecules present in a water droplet, you need to know the chemical formula of water. There is one atom of oxygen, and two atoms of hydrogen in each water molecule, each molecule of water contains three atoms, making the formula H2O.

**Molar Mass of Water**

We can determine the molar mass of water. We can do this by adding up the mass of oxygen atoms and hydrogen atoms in a mole of water by looking at the atomic mass of hydrogen and oxygen on the periodic table. The oxygen mass is 16.00 g/mol, and the mass of hydrogen is 1.008 g/mol, so the mass of one water mole can be calculated as:

mass water = 2 x mass hydrogen + mass oxygen

mass water = 2 x 1.008 + 16

mass water = 18.016 g/mol

In other words, one water mole has a mass of 18.016 grams.

**Density of Water**

We can use the density of water to determine the mass of water per unit volume. The density of water varies depending on conditions (warm water is less dense; cold water is denser), but the value typically used in calculations is 1.00 gram per millilitre (1 g/mL). In other words, 1 millilitre of water has a mass of 1 gram. One drop of water is 0.05 mL of water, so its mass would be 0.05 grams. One mole of water is 18.016 grams, so in 0.05 grams, in one drop, the number of moles is:

- moles of water in one drop = 0.05 grams x (1 mole/18.016 grams)
- moles of water in one drop = 0.002775 moles

**Using Avogadro’s Number**

We can use Avogadro’s number to determine the number of molecules in a drop of water. Avogadro’s number tells us that there are 6.022 x 10^23 molecules of water per mole of water. So, next, we calculate how many molecules there are in a drop of water, which we determined:

- molecules in a water drop = (6.022 x 102^3 molecules/mole) x 0.002275 moles
- molecules in a water drop = 1.67 x 10^21 molecules of water.

In another way; there are 1.67 sextillion water molecules in a water droplet.

Now, the number of atoms in one drop of water is 3x the number of molecules:

- atoms in a drop of water = 3 atoms/molecule x 1.67 x 1021 molecules
- atoms in a drop of water = 5.01 x 1021 atoms

Or, we can say there are about **five sextillion atoms in a drop of water**.

It is Questions Time.

- What is Avogadro Number?
- How many atoms are there in a water drop?
- How do you calculate molar mass of water?
- How many moles of water are there in one drop of water?
- How many water molecules are there in a snowflake weighing 1 mg?