calculate the number of molecules in a drop of water weighing 0.05g

Molecular mass of water = 18 g

No. of moles in 0.05 g of water = 0.05/18

   = 2.78 x 10^-3 mol

One mole = 6.023 x 10²³ molecules

Therefore, 2.78 x 10^-3 mol of water contain = 2.78 x 10^-3  6.023 x 10²³ molecules

  = 1.67 x 10²¹ molecules