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

  • 21
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