Laboratory Record (continued) Determination of the freezing point of pure t-butyl alcohol: Determine the average temperature in the section of the temperature-time curve where the temperature is leveling off. See Figure 2a. Also see Appendix A-4 for application of the Statistics function of the LabQuest 2. Record the start and end times for the region selected and the mean temperatures obtained. std. dev. Mean Temperature (°C) 25.6 Run ID (°C) 1.522 O.998 Run 1 oni 15.1 Run 2 Average Temp. 25.3 Record the average temperature on the previous page. Determination of the freezing point of the unknown/t-butyl alcohol mixture Refer to Figure 2b. Find the linear equation for the early steeply sloped portion by selecting that region, hitting Analyze/Curve Fit/Temperature and then choosing Linear from the drop down menu. Then choose OK. Record the slope (m) and intercept (b) in the table below as well as the start and end times of the selected region. Carry out a similar process for the more gently sloped portion at the longer times. Record its information in the table below. Carry out these operations for both of the runs of the mixture and the pure t-butyl alcohol. The point required is where these two lines intersect. That can be determined by using the straight line equation (Temp =m(time)+b) and realizing that the “x" and "y" values are the same where they intersect. If one equation is given by Temp = m|(time) + bị and the other by Temp = m2(time) + b2, the two equations can be solved simultaneously to give: %3D Temp = (m2b1 – mīb2)/(m2 – m1). In our case, Temp is the temperature where these two lines meet – the freezing point of the mixture. Gently sloped portion of curve Steeply sloped portion of curve Calculated Freezing Intercept (b2) Temperature (°C) Run Slope (m1) Intercept (bi) Slope (m2) -0.136 35,879 -0.0097 15.726 Run 1 Run 2 -0,05 2,23 Fo.0129 17.51) Average Freezing Temperature: Freezing Point Depression: Determination of the Molar Mass of an Unknown Substance Laboratory Record s0.33? #4 %23 Unknown Identifier (All unknowns are nonelectrolytes, so i=1) Determination of the freezing point of pure t-butyl alcohol: (Record time-temperature data on the next page) 29.62 1. Mass of empty test tube 2. Mass of test tube and t-butyl alcohol after determination of freezing point of pure t-butyl alcohol కం.25 20,71 3. Mass of t-butyl alcohol Determination of the molar mass of an unknown: (Record time-temperature data on the next page) 289 52.258 4. Mass of test tube, t-butyl alcohol, unknown 2.258 5. Mass of the unknown 6. Print one graph with the statistics for your group of the t-butyl freezing point determination that shows both runs. Also print one graph for your group of the unknown freezing point that shows both runs. °C 7. Average freezing point of solvent (worked out on next page): °C Average freezing point of solution (worked out on next page): 8. °C 9. ATfbetween t-butyl alcohol and mixture 4. has a molar mass of approximately My unknown # Work for molar mass calculation: CouR -43- Freezing Point Depression: Determination of the Molar Mass of an Unknown Substance