Question

Calculate the number of water molecules in one drop of water whose mass is 0.045 gm

Answer 1

According to the definition of the mole of a substance, one mole of water contains Avogadro's number of water molecules : 6.02214076 × 10²³ mol^-1. The number of the moles of water in 0.045 g of water can be calculated using its molar mass, 18.02 g/mol:

$n = \frac{m}{M} = \frac{0.045}{18.02} = 0.002497$ mol.

Therefore, the number of water molecules is:

$N = n·N_A = 0.002497·6.02214076·10^{23}$

$N = 1.504·10^{21}$ .

Answer: In on drop of water whose mass is 0.045 g , there are 1.504 × 10²¹ molecules of water.