226 3· The freezing point depression constant of water is a rather small 1.86C k

Question

226 3· The freezing point depression constant of water is a rather small 1.86C kg/mol. Addition of 0.100 g of an unknown compound to 2.50 g of water results in a decrease in the melting point of 2.3°C, what is the molecular mass of the unknown? a. molecular mass Addition of 0.454 g of zinc chloride to 333 g of water lowers the melting point by 5.2°C Calculate the expected melting point for this mixture and account for any difference. b. calculated melting point What are some of the problems with the use of water as a solvent for determining the molecular mass of an unknown using the freezing point depression method? c.

Explanation / Answer

a)

T0 - Tf = Kf * molality of solution

T0 = freezing temperature of water

Tf = freezing temperature of solution

Kf = freezing depression constant

2.3 = 1.86 * mass of solute/molar mass of solute * 1000/mass of solvent in g

0-2.3 = 1.86 * 0.1/m*1000/2.5

m = molar mass of the compound = 32.35 g/mol

b)

T0 - Tf = Kf * molality of solution

T0 = freezing temperature of water

Tf = freezing temperature of solution

Kf = freezing depression constant

0 - Tf = 1.86 * mass of solute/molar mass of solute * 1000/mass of solvent in g

0 - Tf = 1.86 * 0.454/136.3 * 1000/3.33

Tf = -1.8605 degrees Celecius

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