What is the molecular weight they determined for this compound ?

Question

The freezing point of water, H₂O, is 0.000 °C at 1 atmosphere. K_f(water) = -1.86 °C/m

In a laboratory experiment, students synthesized a new compound and found that when 10.98 grams of the compound were dissolved in 245.4 grams of water, the solution began to freeze at -0.243 °C. The compound was also found to be nonvolatile and a non-electrolyte.

What is the molecular weight they determined for this compound ?

_______ g/mol

Answer 1

∆T = imK

-0.243 = (1)(m)(-1.86)

m = molality = moles / kg = 0.1306 mol / kg

moles = 0.1306 mol / kg x 0.2454 kg = 0.03206 moles

molar mass = g / moles

molar mass = 10.98 g / 0.03206 mols

molar mass = 343 g/mol (3 sig. figs.)