Solvents \'R Us checks the purity of its solvents before selling them by determi

Question

Solvents 'R Us checks the purity of its solvents before selling them by determining the freezing point of each lot. Pure carbon tetrachloride, CCl_4, freezes at -23.00 degree C and has a K_f of ?9.8 degree C/m. The latest lot has a freezing point of -23.43 degree C. What is the molality of impurities in the solvent? If the impurity is largely hexachloroethane, C_2CL_6 how many grams of this impurity are there per kg of CCl_4? Once the mass of the impurity is determined, calculate the mass % purity: Mass % purity = 100 [1000 g solvent/1000. g solvent + g impurities per kg solvent]

Explanation / Answer

We know the depression in freezing point is calculated as follow:

depression in F.P. = Kf x molality

given depression in freezing point = 0.43 C

Kf = 29.8 K Kg/mol

thus molality of solution = Delta Tf/ Kf

= 0.43/29.8

= 0.0144m

Now assuming the impurity is hexachloroethane ,

molality of impurity = moles /Kg of solvent

= (mass / molarmass)/mass of solvent in Kg

0.0144 = (x g/ 237 g/mol) /(1/1Kg)

thus mass of impurity = 3.42 g

Hence mass % purity = 100 x( 1000/1003.42)

= 99.66%

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