From your plots and the intersection fo the lines drawn, list the melting points

Question

From your plots and the intersection fo the lines drawn, list the melting points you found:

Trial 1 Pure BHT - 76.5

Trial 2 Pure BHT - 60.9

Trial 3 BHT + dichlorobenzene - 74.3

Trial 4 - BHT and unknown - 50.6

10.02g of BHT

.971g of Dichlorobenze

.955g of Unknown

1. Calculate the Kf for BHT from your data.

2. Calculate the molality of your unknown solution using your data and the Kf found in the previous question.

3. Calculate the molar mass of your unknown using your data and the molality of the unkown solution found in the previous question.

Explanation / Answer

Melting Points

Trial 1: Pure BHT 76.50 c

trial2: Pure BHT 76.80 c (Average of two trial = 76.65 C)

trial 3: BHT+ dichlorobenzene 72.0 c

trial :4 BHT and unknown 71.4 c

BHT: 10.005
dichlorobenzene: 0.602 g

You need to calculate the molality of the dichlorobenzene solution:
Mass of dichlorobenzene in 1.0kg of BHT 0.602 /10.005*1000 = 60.1699g in 1.0kg BHT
Molar mass dichlorobenzene = 147.01 g/mol
Molality of solution = 60.1699g /147.01 = 0.409m solution

This decreased the freezing point by 4.65°C
Kf for BHT = 4.65/0.409m = 11.369°C/m.kg

The freezing point depression for solution #2 was 5.25°C
Therefore the solution was 5.25/11.369 = 0.461 molal

Mass unknown in 1.0kg BHT = 0.660 g /10.005 g *1000 = 65.96 g in 1.0kg

Molar mass of unknown = 65.96 /0.409 m = 161 g/mol

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