Question

Calculate the molar mass of an unknown solid (assume a non-electrolyte), if 1.35 grams of th esolid was dissolved in 15.0 grams of dodecanol and caused a freezing point depression of 2.34

Answer 1

Mass of solid dissolved m1 = 1.35 g
 Mass of dodecanol m2 = 15 g
 Freezing point depression Td= 2.34 C
 Freezing point depression constant for dodecanol K = 2.9 C/mol
 Calculating the molarity of dodecanol m => Td = K x molarity
 => 2.34 = 2.9 x molarity => molarity = 2.34 / 2.9
 Molarity M = 0.807
 We have mass and molarity, calculating for molarity for 15 g solvent
 = 15 x (0.806 / 1000) = 0.0121
 Now calculating the molar mass of solid = m1 / molarity
 => 1.35 / 0.0121 = 111.538
 So the molar mass of the Solid is 111.54