[cabaal]

The freezing point of a solvent is 178.4 °C and the cryoscopic constant is 37.7 °C*kg/mol. How many grams of solute (whose molar mass is 128.2 g/mol) should be mixed with 11.8 g of solvent to lower the freezing point to 169.5 °C?

178.4 °C - 169.5 °C = 8.9 °C Δt.
K_f*m=Δt ==> (37.7 °C*kg/mol)(m)=8.9 °C ==> m = 4.2 mol/kg
4.2 mol/kg = x moles solute/(11.8 g solvent * 1 kg/1000g) ==> x = 0.050 mol solute
0.050 mol solute * 128.2 g/mol = 6.4 g solute

Am I doing anything wrong? My answer was marked incorrect but I can't find any mistakes in my math. Thank you.

[Borek]

(37.7 °C*kg/mol)(m)=8.9 °C ==> m = 4.2 mol/kg

Really?

[cabaal]

(37.7 °C*kg/mol)(m)=8.9 °C ==> m = 4.2 mol/kg

Really?

Oops, that should be 4.2 kg/mol shouldn't it?

[Borek]

Stop guessing, calculate.

[cabaal]

Oh wait...wow how did I even make a mistake so large? That's horrible. 

[cabaal]

Kf*m=Δt ==> (37.7 °C*kg/mol)(m)=8.9 °C ==> m = 0.24 mol/kg
0.24 mol/kg = x moles solute/(11.8 g solvent * 1 kg/1000g) ==> x = 0.0028 mol solute
0.0028 mol solute * 128.2 g/mol = 0.36 g solute

I have absolutely no idea how I made such a stupid mistake.  Thank you for your help, Borek.