Part 1:
Pure Lauric Acid: 8.16g
The freezing point of pure Lauric Acid = 43.9 degrees celsius
Part 2: Mixture
Lauric Acid: 8.19g
Benzoic Acid: 1.05g
Freezing Point: 39.9 degrees C
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Directions:
Read through the following introduction and procedure for the lab for important background information on the measurements involved.
Watch the lab videos in this folder to see the procedure being done, collect your data, and learn more about the measurements. There is also a written version of the procedure directions below which you can refer to as you watch the videos if that is helpful. The data you need can only be found in the videos.
Complete the lab report by following the instructions at the end of this document describing my expectations for the following sections:
a. Introduction
b. Data and calculations
c. Answers to analysis/conclusion questions.
Submit your completed lab report as a word or pdf file by uploading it to the assignment dropbox in the Lab 1 folder on our course blackboard site.
Introduction:
When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. In this experiment, you will measure the freezing point depression of a mixture and use it to determine the molecular weight of the solute in the mixture.
Figure 1
1 Procedure Reference: J. Randall et al., Advanced Chemistry with Vernier, Vernier Software and Technology: Beaverton, OR, 2009, p. 4-1.
PROCEDURE
Part I Determine the Freezing Temperature of Pure Lauric Acid
Obtain and wear goggles.
Connect the Temperature Probe to LabQuest and choose New from the File menu.
On the Meter screen, tap Rate with the stylus. Change the data-collection rate to 1 sample/second and the data-collection length to 600 seconds.
To a 400 mL beaker, add about 300 mL of tap water. Begin heating water on a hot plate as a hot water bath.
In a second 400 mL beaker, again add about 300 mL of tap water with a temperature of 20– 25°C. This will be your room temperature water bath. Place the beaker at the base of a ring stand.
Measure out approximately 8 g of solid lauric acid into a large test tube.
Use a utility clamp to attach the test tube to a second ring stand. Position the ring stand and clamp so that the lauric acid in the test tube is suspended below the level of the water in the hot water bath.
When the lauric acid has completely melted, remove the test tube from the hot water bath (use the utility clamp as a test tube holder) and fasten to the second ring stand so that the test tube is above the room temperature water bath. CAUTION: Be careful not to spill the hot lauric acid on yourself and do not touch the bottom of the test tube.
Insert the Temperature Probe into the hot lauric acid. Start data collection by pressing on the arrow icon in the lower right hand corner of the LabQuest screen.
Lower the test tube into the water bath. Make sure the water level outside the test tube is higher than the lauric acid level inside the test tube, as shown in Figure 1.
With a slight up-and-down motion of the Temperature Probe, continuously stir the lauric acid for the ten-minute duration of the experiment.
When data collection is complete, use the hot water bath to melt the lauric acid enough to safely remove the Temperature Probe. Carefully wipe any excess lauric acid liquid from the probe with a paper towel or tissue.
Lift the test tube out of the hot water bath and allow to cool. When cool, label the test tube as “Pure Lauric Acid” and return to your instructor.
The freezing temperature can be determined by finding the mean temperature in the portion of the graph with nearly constant temperature.
a. Select the data point at the beginning of the flat portion of the graph and drag across the flat portion with the stylus to select the region.
b. Choose Statistics from the Analyze menu.
c. Record the mean (average) temperature as the freezing temperature of pure lauric acid.
Store the data from the first run by tapping the File Cabinet icon.
Part II Determine the Freezing Point of a Solution of Benzoic Acid and Lauric Acid
16. Into a second large test tube, add ~1 g of benzoic acid and ~8 g of lauric acid. Record the precise masses of each substance measured into the test tube in your lab notebook. Repeat Steps 7-12.
Lift the test tube out of the hot water bath and allow to cool. When cool, label the test tube with the precise masses of benzoic acid and lauric acid that you measured into it and return to your instructor.
Freezing Point
Time
When you have completed Step 12, examine the data pairs on the displayed graph. The freezing point of the benzoic acid-lauric acid solution can be determined by finding the temperature at which the mixture initially started to freeze. Unlike pure lauric acid, the mixture results in a gradual linear decrease in temperature during freezing. As you move the examine line, the temperature and time values are displayed to the right of the graph. Locate the initial freezing temperature of the solution, as shown above. Record the freezing temperature in your lab notebook.
To view both sets of temperature data on the same graph, tap Run 2 with the stylus and select All Runs.
The arrow is pointing at the freezing point
Lab Report Instructions:
Below you will find specific instructions for the three sections I want to see in your lab report:
1) Introduction
2) Data
3) Calculations and Analysis
I want to give you this advice on writing lab reports for me first, though. When I open your lab report file(s), I want to see your introduction section first. However, this does not mean that you should necessarily write your introduction first. I highly recommend that you start writing your lab report with the data followed by the calculations and analysis questions. Once you have completed these, you will likely have a better sense of the purpose of the lab and how the concepts you need to define relate to your procedure and data (the essence of your introduction). It is standard course for many scientists when writing a scientific paper on their results to start writing their results and analysis first to make sure they clearly know and present the story their data holds. Then they’ll go back and write the introduction to provide the relevant background concepts for that story. It’s a more efficient way to write lab reports (and scientific papers) and itusually produces much better and more cohesive end products.
For your lab on this topic, I want to see the following:
1) An introduction section:
This should be a clearly written and cohesive introduction with several paragraphs that introduces and explains the major concepts underlying your lab. For this particular lab, I am looking for:
A clear purpose statement for the lab. What was the objective of the experimental procedure shown? Clearly define the solute and the solvent in this experiment as well.
A clear explanation of the concept of colligative properties in general and freezing point depression specifically. What are the factors that freezing point depression (or any colligative property) are influenced by?
A clear explanation the formula for freezing point depression and all the variables involved. Since this lab is about determining molar mass for the solute, make sure to explain how molar mass can be related to the freezing point depression formula.
Written in paragraph form with complete grammatically correct sentences. Paragraphs should logically connect from one to the other.
2) Data Tables:
Organize and present the data given in the videos in appropriate data tables. I expect the following:
Use the tables function in your word processing program to create the tables and include all relevant data in an organized manner.
Include column and row headings that clearly explain what the data represents
Include units for all measurements (as part of the column or row heading or with
each individual data point)
Make sure you’ve recorded the correct number of significant figures for each data
point.
3) Calculations and Analysis:
Complete the requested calculations and answer the associated analysis questions (below). As you do the requested calculations and answer the analysis questions, please bear the following in mind:
I want to see your set up for all calculations. Show the formulas used, the data substituted in for each variable in the formula, and, where appropriate, how you rearranged to solve for the final, requested answer. The more you show me of your calculations, the better your chances of earning more partial credit if you make a calculation error!
You are allowed to handwrite your calculations and upload separate pictures (or insert the pictures into your lab report file). Please make sure your calculations are neatly and clearly organized. Label each requested calculation so I know what I’m looking at.
Answer all questions that require a text answer in complete, grammatically correct sentences that make it clear what question you are answering!
Calculations And Analysis Questions to Answer
Calculate the difference in your measured freezing points between the pure lauric acid and your solution of benzoic and lauric acid (Tf).
Use the value of Tf you calculated above to determine the molality (m) of benzoic acid in dissolved in your solution, in mol/kg, using the formula Tf = Kf m. The Kf value for lauric acid is 3.9°C•kg/mol.
Calculate moles of benzoic acid solute, using the molality above and the recorded mass (in kg) of lauric acid solvent in the solution test tube.
Use the moles of benzoic acid determined above and the recorded mass (in g) of benzoic acid solute in the solution test tube to calculate the experimental molar mass of benzoic acid, in g/mol.
???????????????????????? ?????????? ???????? = ???????? ???? ?????????????? ????????/???????????????????? ?????????? ???? ?????????????? ????????
Determine the accepted molar mass of benzoic acid from its formula, C6H5COOH.
6. Calculate the percent error between the experimental and accepted values.
% ?????????? = |???????????????? ?????????? - ???????????????????????? ??????????| × 100 /???????????????? ??????????
7. Analysis questions. Answer in complete sentences that make it clear what question you are answering.
a) If a small amount of the benzoic acid measured fails to dissolve in the lauric acid, would you expect the reported freezing point to be too high or too low? Why?
b) If a small amount of the benzoic acid measured fails to dissolve in the lauric acid, what impact would this have on the molecular weight calculated? Would it be too high or too low? Explain your answer.
c) Why might your value for molecular weight be different than the accepted value? Please be specific, thoughtful, and thorough in your discussion by providing at least three potential sources of error and uncertainty in this experiment. Explain how they might have impacted your measurements and your calculations of molality and molecular weight. Simply stating ‘Human error’ or ‘calculation error’ are never acceptable answers.
d) If you were to repeat this experiment, what might you do to improve your results?
