The number of molecules in a drop of water (0.0018 ml) at room tempera

1.	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» (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.

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»

(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.

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.