Question

Plot cooling curves for the p-dichlorobenzene solution in cyclohexane. Since the freezing plateau isn’t strictly horizontal, extrapolate the freezing point graphically.

Data:

Time (s)	Temperature (°C) Run 1	Temperature (°C) Run 2
0	20.1	20.9
20	10.3	18.8
40	7	10.1
60	3.8	7
80	1.3	4.8
100	-0.3	3
120	-1.2	2.2
140	-1.7	1.7
160	-1.2	1.2
180	-1.6	0.5
200	-1.8	-0.3
220	-1.9	-1.2
240	-2.5	-0.8
260	-2.1	0.4

Answer 1

For this, we plot temperature vs time in two series, one in steep slope, and anothe in almost horizontal slope, so we can see the change and get the freezing point in the point of inflection. The procedure in excel is as follows: First you make both plots for bothe slopes in both runs, then you add trendline, and extrapolate it so that both can cross, and determine the inflection point.

For run 1, and to get exact results, we equalize both expressions obtained:

x = (y - 17.32) / -0.2205

y = [-0.0101 * ((y-17.32)/-0.2205)] + 0.2261

y = -0.59 ºC

For run 2, we apply same procedure:

y = [-0.0223 * ((y-21.12)/-0.22)] +4.7494

y = 2.9ºC