Question

caffeine the freezing point of a solution prepared by dissolving 150 x 10^-3 g of caffeine in 10.0 g of camphor is lower by 3.07C than that of pure camphor (Kf = 39.7 C/m) what is the molar mass of caffeine? Elemental analysis of caffeine yields the following results: 49.49% C, 5.15% H, and the remainder is 0. What is th molecular formula of caffeine? Please explain the steps taken to get both answers. Thank you

Answer 1

Answer – Given, mass of caffeine = 0.150 g, mass of camphor = 10.0 g ,

∆Tf = 3.07^oC , Kf = 39.7^oC/m

We know ,

∆Tf = Kf* m

So, m = ∆Tf / Kf

         = 3.07^oC / 39.7^oC.m^-1

        = 0.0773 m

We know, molality = moles of solute / kg of solvent

So, moles of caffeine = molality * kg of solvent

                                   = 0.0773 m * 0.010 kg

                                   = 7.73*10^-4 moles

Molar mass of caffeine = 0.150 g / 7.73*10^-4 moles

                                      = 193.9 g/mol

For the molecular formula we need the detail percent of the C, H and N or O at least. First we need to calculate percent of each element like total is 100 %

Then assume the 100 g of sample, so mass percent of each element is its mass

Then calculated moles of each and look for the smallest moles. Then divided by this mole to each moles and then will get the empirical formula. The calculate molar mas of empirical formula and then , n= molecular mass / empirical formula mass and finally , molecular formula = n * empirical formula.