Question

An aqueous solution containing 17.5 g of an unknown molecule (non electrolyte) compound in 100 g water has a freezing point of -1.8 degrees Celsius. Calculate the molar mass of the unknown compound

Answer 1

An aqueous solution containing 17.5 g of an unknown molecule (non electrolyte) compound in 100 g water has a freezing point of -1.8 degrees Celsius. Calculate the molar mass of the unknown compound.

For water, K_f = -1.87 ^oC Kg/mol

17.5 g solute present in 100 g water

=> In 1000g water, 175 g solute present

Molality, m = (temperature difference/K_f ) = T /K_f = (-1.8-0)/-1.87 = 3.366 m

Molality is the miles of solute in 1000g of solvent

=> number of moles/1000 g water = weight of solute in 1000gm water/molar mass of compound = 3.366 m

=> molar mass of compound = 175/3.366 = 51.99 g/mol