Given that the value of kf for water is 1.86°c/m, what is the freezing point of an aqueous solution of nacl prepared by mixing 427.0 g solute with 1.000 kg of water

Answer is: the freezing point of an aqueous solution of sodium chloride is -13.6°C. 
m(NaCl) = 427.0 g.
n(NaCl) = m(NaCl) ÷ M(NaCl).
n(NaCl) = 427 g ÷ 58.4 g/mol.
n(NaCl) = 7.31 mol.
b(NaCl) = n(NaCl) ÷ m(H₂O).
b(NaCl) = 7.31 mol ÷ 1.00 kg.
b(NaCl) = 7.31 m.
ΔT = Kf · b(NaCl).
ΔT = 1.86°C/m · 7.31 m.
ΔT = 13.596°C.

The answer is c. 320 g