Question

Volume of drop of water is 0.05 ml and the density of water is 1g/ml.how many water molecules are present in a drop of water

Answer 1

You must know the density of water is about 1g/ml. 
.05 ml=Xgrams when 18g is one mole 
Since you have .05 ml= about 0.05 g 
divided by18g = number of moles 
=2.78*10^-3 moles multiply by 6.02*10^23 =number of molecules

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