[random87]

The freezing point of pure benzene is 5.53±0.02 °C and its enthalpy of fusion is
127.4±0.1 J g-1. A solution containing 6.0547±0.0005 g of an unknown solute in
1000.0±0.5 g of benzene has a freezing point which is 0.13±0.02 °C below that of
pure benzene. Calculate the molecular weight and the uncertainty of the
molecular weight of the unknown solute using the method of propagated error.
Assume ideal behavior of the solution

[MrTeo]

The rules are pretty easy, you only need to apply them:

http://en.wikipedia.org/wiki/Propagation_of_uncertainty