What is the molecular weight of the unknown compound?

Question

When 2.25 grams of an unknown non-electrolyte dissolves in 12.0 grams of water, the solution freezes at -3.16 °C. What is the molecular weight of the unknown compound? Kf for water = 1.86 °C/m. 

Answer 1

ΔT_f = k_f*i*m => m = ΔT_f/k_f*i

=> m = n_solute/kg_solvent

n_solute/kg_solvent = ΔT_f/k_f*i => n_{solute =} ΔT_f*kg_solvent/k_f*i

=> MW = mass/n_solute => n_solute = mass/MW

mass/MW = ΔT_f*kg_solven/k_f*i_t => MW = mass*k_f*i/ΔT_f*kg_solvent

<=> (2.25g)((1.86ºC*m^-1)(1)

(3.16ºC)(0.012kg)

MW = 110.4g*mol^-1

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