Considering that 1ml of water has a mass of 1gram, calculate moles of 1drop.
20drops = 1ml
then 1drop = 1×1/20
= 0.05ml
therefore if 1ml =1g
then 0.05ml = 0.05g(mass of 1drop of water)
molar mass of H2O = 18
Hence its moles = 0.05/18
=2.778 × 10^-3 molecules
If 1mole = 6.023 × 10^23 molecules
then 2.778×10^-3 =
6.023×10^23 × 2.778 ×10^-3 molecules
= 1.6731 × 10^21molecules(in a drop of water)

**Number of Molecules in a Drop of Water**

To determine the number of molecules in a drop of water, we need to consider the molar mass of water, Avogadro's number, and the volume of a drop.

**1. Molar Mass of Water**
Water has a molar mass of approximately 18 grams per mole. This means that one mole of water contains 6.022 x 10^23 molecules (Avogadro's number).

**2. Volume of a Drop of Water**
The volume of a drop of water can vary depending on factors such as temperature, surface tension, and dropper size. However, an average drop is considered to be approximately 0.05 mL.

**3. Converting mL to moles**
To determine the number of moles in a drop of water, we need to convert the volume of the drop (in mL) to moles. We can use the molar volume of water, which is approximately 18 mL per mole at standard temperature and pressure (STP).

To convert 0.05 mL to moles, we divide by the molar volume:
0.05 mL ÷ 18 mL/mol ≈ 0.00277 mol

**4. Calculating the Number of Molecules**
Now that we have the number of moles in a drop of water, we can use Avogadro's number to calculate the number of molecules.

Number of molecules = Number of moles x Avogadro's number
Number of molecules = 0.00277 mol x 6.022 x 10^23 molecules/mol ≈ 1.667 x 10^21 molecules

Therefore, there are approximately 1.667 x 10^21 water molecules in a drop of water.

**Conclusion**
In conclusion, the number of molecules in a drop of water is approximately 1.667 x 10^21 molecules. This calculation is based on the average volume of a drop of water and the molar mass of water. It is important to note that this is an approximation and the actual number of molecules may vary slightly depending on the specific conditions and size of the drop.