What is the number of water molecules present in a tiny drop of water (volume 0.0018 ml) at room temperature?

To find the number of water molecules in a 0.0018 mL drop, we first convert volume to mass. At room temperature, the density of water is approximately 1 g/mL [c1], making the mass of the drop 0.0018 g [t1, t3]. The molar mass of water (H₂O) is 18 g/mol [t2, t3]. The number of moles is calculated by dividing the mass by the molar mass: 0.0018 g / 18 g/mol = 0.0001 moles (or 10⁻⁴ moles) [t1, t3]. Using Avogadro's number (6.023 x 10²³ molecules/mol), the total number of molecules is determined by multiplying the moles by this constant: 10⁻⁴ × 6.023 x 10²³ = 6.023 x 10¹⁹ molecules [t1, t3, t9]. Thus, a tiny drop of 0.0018 mL contains 6.023 x 10¹⁹ water molecules.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 9: The Amazing World of Solutes, Solvents, and Solutions > Conversion factor for density > p. 141