Question

In the course of research, a chemist isolates a new compound with an empirical formula C3H3O2. 4.97 g of the compound when dissolved in 100. g of water produces a solution with a freezing point of −0.325°C. What is the molecular formula of the compound? (For water, Kf = 1.86°C/m.) (For your answer, enter the integer that tells the number of multiples of the empirical formula present in the molecular formula.)

Answer 1

You can find the molal concentration (m) from 0.325degC/(1.86degC/m)

m=(number of moles)/0.1 kg

hence find number of moles.

molecular mass = (mass used)/(number of moles)= 4.97g/(number of moles).

RMM of C3H3O2 = 71

multiple = molecular mass/71

As a point of interest a compound that only contains C, H and O must have an even numebr of hydrogen atoms. Hence the multiple must be an even number.