[calvin coolidge]

Problem: A solution containing 0.200g of an unknown substance in 2.50g of cyclohexane (C6H12) is found to freeze at 5.1 degrees C. What is the molar mass of the unknown substance if its a non-electrolyte?

Given for cyclohexane: Kf=-20.0, freezing pt=6.55. I'm going to use the formula: DeltaT= (i)(Kf)(m). I know i=1 since its a non-electrolyte and deltaT will be 5.1-6.55. I guess I'm stuck on finding the molality (m). I have the grams of unknown, but not the moles. How do I get the moles in order to set it up for the molality (moles of solute/kg of solvent)?

Thanks for any help you can provide.

~CC

[rabolisk]

You seem to be going backwards. You're not converting mass to moles to molality. You're going from molality to moles to molar mass.

DeltaT = i(Kf)(m) is enough to give you molality, which is moles of solute/kg of solvent. You have the mass of solvent, so you can find moles of solute, and then finish out the rest of the problem.

[calvin coolidge]

Thanks for setting me straight, rabolisk! After staring at a problem for awhile, I have a habit of making it harder than it needs to be....

Thanks for the quick response too!

~CC

[rabolisk]

No problem. Now can a bro get some mole snack?