Chemistry How to get molality without using a molecular formula?

[sp3sp2sp]

Homework Statement

Freezing point depression question[/B]
student determines molar mass of acetone using the method used in this experiment. She found that the eq temp of water and ice was 1 deg C. She then adds 11.1g of the sample to mixture and temp falls to -3 deg C. Mass of solution was 90.4g
What is the molality of the acetone using the experimental data and equation delta T_f = mk_f?

Homework Equations

Im lost on how to get moles without using the MM from formula for acetone?

The Attempt at a Solution

I know that T_f = 4deg C
also that m = moles solute / kg solvent
Also (if this is useful) that mass % = 12.28% acetone
also that kg of the solution will be = .904kg
thanks for any help

 

[mjc123]

You know ΔT_f, you know (or can look up) k_f, so you can work out m. If you also know the MW of acetone, you can check how accurate this m value is.