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 , Volume = 0.0018 ml
Mass = $Density\times volume$
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 = $\frac{givenmass}{molarmass}=\frac{0.0018}{18}=0.0001mol={10}^{-4}moles$

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