What is the freezing point of a two Molar solution of sodium chloride?

Answer 1

Actually you have an electrolyte, so the drop in the freezing point is

ΔT=i*Kf*m

where i is the van't Hoff coefficient and (for simplicity) for strong electrolytes it is equal to the number of ions that are produced by its dissociation, according to the molecular formula.
NaCl dissociates completely to two ions (Na+ and Cl-) thus i=2

for water Kf=1.86

m is the molaLity and not molaRity.
Your solution is 2 M = 2mole solute/L of solution
molality is mole solute/kg of solvent. You need additional info in order to convert molarity to molality

If you had 2 molaL and not molaR solution then m=2, or if we assume due to lack of more info that in this case molaRity=molaLity...

ΔT=i*Kf*m =2*1.86*2 = 7.44
For pure water Tfwater=0 C, so for the solution
Tf= Twater-ΔT= 0-7.44= -7.44 C

Answer 2

Two molar solution IN WHAT? I assume water. If so, you'll need the molal freezing point depression constant (Kf) in water.

dT = Kf * m

where dT = freezing point depression, Kf = freezing point depression constant (i.e. the slope of the freezing point curve) and m is the molality, in this case, 2 M.

Kf is specific to solvent.

This is a classic gen chem lab experiment and is usually done with varying masses of solute to determine the freezing point depression due to a certain solute (as a fcn of mass).

Gimme Kf and you get your answer.