The normal freezing point of a certain liquid X is -1.1 °C, but when 38.6 g of urea ((NH$_2$)$_2$CO) are dissolved in 600. g of X, it is found that the solution freezes at -2.8 °C instead. Use this information to calculate the molal freezing point depression constant $K_f$ of X. Be sure your answer has the correct number of significant digits.