Chemistry
posted by Karolina .
The normal freeing point of cyclohexane if 6.6 degrees Celsius. A 0.2g sample of an unknown solute is dissolved in 50 ml of cyclohexane (d0.8g/ml, Kf=20 degreesCelsius/m. If the freezing point of the solution is 3.6 degrees Celsius, what is the molar mass of the solute?

delta T = Kf*molality
Solve for molality.
molality = moles/kg solvent. Use the density to convert 50 mL solvent to mass.
Solve molality equation for moles.
moles = grams/molar mass. You have moles and grams, solve for molar mass. 
what grams do i use to solve for "moles = grams/molar mass?

It's the mass of the unknown that you want to determine the molar mass of. That is 0.2 g sample from the problem.