Question

The number of water molecules present in a drop of water (volume 0.0018 ml) at room temperature is

Answer 1

density of water = 1 g/ml
mass of 0.0018 ml of water = 0.0018×1 = 0.0018 g
molar mass of water = 18 g/mol

number of moles in 0.0018g of water = $\frac{0.0018}{18}=10^{-4}\ mole$

number of water molecules in 1 mole = 6.022×10²³ molecules
number of water molecules in $10^{-4}$ mole = $10^{-4}*6.022*10^{23}=\boxed{6.022*10^{19}\ molecules}$