How would the molar mass determination of a strong electrolyte ionic compound by

Question

How would the molar mass determination of a strong electrolyte ionic compound by the freezing point depression procedure of this experiment be effected by each of the following scenarios? The researcher assumed that alpha = 1 at a concentration 0.1 m. The cooling curse for the solvent was analyzed correctly, but the minimum temperature of the supercooled region of the cooling curve was used to determine the freezing point of the solution. The thermometer was incorrectly calibrated and consistently showed a temperature 0.08 degree C higher than the actual temperature for all measurements. A sample of potassium chloride is contaminated with a small amount of sodium bromide. (Assume that the measurements were taken under conditions where alpha = 1 for both compounds) The distilled water used in the experiments was contaminated with a very low concentration of magnesium chloride.

Explanation / Answer

Q1.

a)

if activity is assumed to be 1 at molality of 0.1 m , then this will have no effect, since molality is low, therefore the activity is low. No effect in solution

b)

The dT is incorreclty measured, therefore it will be lower than normal

This means, MW will be higher than that of the real value

c)

if T is 0.08°C higher, then the actual value will be :

dT = -Kf*m

m = mol of K /kg sovlent

mol of K = mass/MW

so.. if dT is high, then dT is lower, meaning MW will be "incorrectly" higher than the actual value

d)

KCl is contaminated with NaBr:

The molar mass of KCL = 74.5513 for NaBr = 102.894

so, the molar mass will be higher than the acutal vlaue due ot NABr presence

e)

if MgCl2 is present, then there is a higher "i" value, i= 3

so

The MW must be lower than expected

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