Blog: 1412 Lab

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Lab B – Freezing Point Depression (Exp 12)

Determination of Molecular Weight using Freezing Point Depression.

This lab has two files to upload... see filename checklist... the graph must also be in PDF format.

Do NOT use the accepted or textbook MW for benzoic acid (BA) in any of the Line 6–9 calculations. MW is what you are trying to determine! The whole thrust of the lab is to determine the MW from FP Depression and colligative properties.
The only place the accepted value for benzoic acid is used is in Line 10, in which you compare your experimentally-determined benzoic acid MW to the accepted benzoic acid MW.
In your calculations, pay attention to significant figures.

Line 6: Detn ∆T from the difference of the flat, or parallel, portions of the two curves.
Line 7: ∆T = m • Kf, so m = ∆T/Kf
Line 8: molality = mol,solute / Kg, solvent, so... mol,BA = molality • Kg,LA
Line 9: MW = g/mol. You measured mass of BA earlier, and you calculated moles of BA on Line 8.
Line 10: Use formula for % error. Remember to report you answer to the correct number of sig figs, and the correct unit.
The percent difference formula is:
% error = | measured value — accepted value | × 100 / accepted value

From the video and data set provided, you will generate a total of three graphs.
Of the three graphs, only one has useful data. You will select the useful graph, and "mark it up" using the Logger Lite features to generate the needed data.
(Caveat! If you select the wrong graph from which to pull your data, almost all of your calculations will be incorrect.)
The graph, and data from the graph, is then used to answer the postlab questions.

Postlab Q1: You must be aware of the general features of the three types of graphs (0th-order vs. 1st-order vs. 2nd-order). Refer to your textbook and videos for detailed explanations.
Postlab Q2 & Q3: You will use Logger Lite to generate a linear equation for the relevant curve ("graph") you selected to use. Understand what each part of the equation represents to answer the questions.
Postlab Q4: select two points on your curve, and manually determine the 1/2-life.
Postlab Q5: calculate the 1/2-life using the appropriate 1/2-life equation.
Postlab Q6: Use the % Difference to calculate the difference between your Q4 and Q5 values. The percent difference formula is:
% difference = | value 1 — value 2 | × 200 / (value 1 + value 2)
where 200 is an exact number. Pay attention to significant figures.