Question

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

(a) 6.02 x 10²³ 

(b) 1.084 x 10²³ 

(c) 4.84 x 10²³  

(d) 6.02 x 10²³ 

Answer 1

(a) 6.02 x 10²³ 

0.0018 ml – drop of water = 0.0018 g 

H₂O = molecular mass = 18 g. 

Number of molecules in 18 g = 6.023 x 10²³ 

∴ Number of molecules in 0.0018 g 6.023 x 10²³ x 0.0018 

= 6.023 x 10²³ x 10⁵

= 6.023 x 10¹⁹ molecules.

or

Density of water at 25°C = 997.0479 g / L 

Mass of 0.0018 ml (or) 0.00 18 x 10^-3 L 

= D x V 

= 997.05 x 0.0018 x 10^-3 

= 1.795 x 10^-3 g 

Molar mass of water = 18 g 

Mole = Mass/ (Molecular mass) 

= (1.795 x 10^-3) / 18

= 9.971 x 10^-5 

∴ Number of molecules in 0.0018 ml = moles x (Avogadro number) 

= 9.971 x 10^-5 x 6.023 x 10²³ 

= 6 x 10¹⁹ molecules.