Answer to Question #32541 in Inorganic Chemistry for imran

Question

a solution is prepared from 2.4gram of a organic compound and 75 gram of benzene. if the freezing point of the solution is -0.975 centigrade.what will be the molecular formula of the organic compound if the organic compound is made of carbon and hydrogen.

Answer 1

Theformula for freezing point depression (for non-electrolytes like sucrose) is
ΔT_f = K_f * m, where ΔT_f = change in freezing point, K_f = molal freezing pointdepression constant for the solvent, and m = molality of the solution.
freezing point of benzene is 5.5 ^oC
ΔT_f = 5.5 + 0.975 = 6.475
The K_f for benzene is 5.12 degrees Celsius C/m.
m = ΔT_f / K_f;
m = 6.475 / 5.12 = 1.26
The molality of the solution:
molality = moles solute / mass (kg) solvent
moles solute = molality * mass (kg) solvent
moles solute = 1.26 * 0.075 kg = 0.095 moles
molar mass = mass / moles
molar mass = 2.4g / 0.095 moles = 25.26 g/mol
molecular formula of the organic compound is C₂H₂