Question

calculate the molar mass? calculate the value of the freezing point depression constant, Kf, for acetic acid.

	Trail 1	Trail 2
Mass of acetic acid	1.735g	2.195g
Mass of acetylsalicylic acid	0.06g	0.09g
Freezing point	14.247C	14.247
Change in temperature	0.852C	0.852C

Answer 1

Her the mass of acetic acid is more so it acts as solvent.

Mass of acetic acid , M = average mass in trail 1 & trail 2

                                  = (1.735+2.195) / 2

                                 = 1.965 g

                                 = 1.965x10^-3 kg

Mass of acetylsalicylic acid is , M' = average mass in Trail 1 & Trail 2

                                                  = (0.06 + 0.09 ) / 2

                                                  = 0.075 g

Formula of acetylsalicylic acid is C₉H₈O₄

So molar mass of C₉H₈O₄ is = (9x12) + (8x1) + (4x16) = 180 g/mol

We know that ΔT _f = K_f x m
Where

ΔT _f = depression in freezing point

        = freezing point of pure solvent – freezing point of solution

        = 0.852 ^oC

K _f = depression in freezing constant = ?

m = molality of the solution

    = ( mass of C₉H₈O₄/ Molar mass ) / weight of the solvent in Kg

   = (0.075 / 180 ) / ( 1.965x10^-3 )

   = 0.212 m

Plug the values we get

0.852 = K_f x 0.212

     K_f = 4.02

Therefore the freezing point depression constant is 4.02