Freezing point depression can be used to experimentally determine the van \'t Ho

Question

Freezing point depression can be used to experimentally determine the van 't Hoff factor of a solute in solution. Given the data in the table, answer the questions below and determine the "real" van't Hoff factor of the solute. What mass of solute was used? What is the freezing point depression Delta T_f of the solution? What is the colligative molality mc of the solution? How many moles of solute particles are present in solution? If the solute has a molar mass of 120.68, what is the van't Hoff factor i for the solute? (Remember that experimentally,/does not have to be an integer.)

Explanation / Answer

a)

mass of soltue = mass of solution - mass of solvent

so

mass of solute = 8.794 - 7.975

mass of solute = 0.819 g

b)

dTf = 0- (-4.97)

dTf = 4.97 C

c)

we know that

dTf = Kf x mc

so

4.97 = 1.86 x mc

mc = 2.672

d)

now

molality = moles of solute particles x 1000 / mass of solvent (g)

so

2.672 = moles of solute particles x 1000 / 7.975

moles of solute particles = 0.02131

e)

now

moles of solute particles = i x mass of solute / molar mass

so

0.02131 = i x 0.819 / 120.68

i = 3.14

so

the value of i is 3.14

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