Question

Calculate the molarity and molality of 20% aquaous ethanol solution by volume ( density of solution = 0.960 gm pr cm cube ) .Assume the solution to be ideal??

Answer 1

20% aqueous ethanol solution by volume means 20 ml/ ethanol in 80 ml/ water.
Density of the solution as 0.960 g/cm³ or 0.960 g/ml
Now,
Mass of solution = density x volume
= 0.960 x 100
= 96 g
Mass of 80 ml water = density of water x volume of water
= 1g/ml x 80 ml = 80 g
Mass of ethanol in solution = mass of solution - mass of water
= 96-80 = 16 g
Molar mass of ethanol,(C₂H₅OH) = 12 x 2 + 5x 1 + 16 + 1
= 46 g
Number of moles of ethanol = 16/46 = 0.348
Molarity = number of moles/volume in litres
= 0.348/0.1
= 3.48 M

Molality = number of moles of solute/ mass of solvent in Kg
= 0.348/0.08
= 4.35 m