Question

ASAP! PLZ I need all answers for pre lab question in Molecular Weight Determination by Freezing Point Depression

Prelab questions:

(PL1) What is the molar mass of X (i=1) if 50.00g of water containing 2.50g of X has a freezing point of -11.5C?

(PL2) Using information from PL5, recalculate the molar mass of X if i=2.

(PL3) Compare your answers from (PL5) and (PL6). Briefly explain.

(PL4) How is melting point different than freezing point?

(PL5) List the following in the increasing melting points. Pure water, 1.0M sugar water, 1.0M salt water.

Answer 1

PL1

MW if

dTf = -Kf * m * i

m = mol/kg solvent = (mass/MW) / kg solvent

then

dTf = -Kf*( (mass/MW) / kg solvent) * i

-11.5 = -1.86*(2.5/MW) / (50/1000) * 1

MW = -1.86*2.5 / ((50/1000) * 1) / -11.5 = 8.08695 g/mol

PL2

for i = 2

MW = -1.86*2.5*2 / ((50/1000) ) / -11.5 = 16.1739 g/mol

PL3

there is an increase due to the dissociation factor "i" of vant Hoff

PL4

melting --> solid to liquid, interactions are lower

freezing --> liquid to solid, interactions are stronger

PL5

Pure water < 1 M sugar (i = 1) < 1M Salt (i= 2)