Instructions and guidance for Experiment 34(Deicers and Antifreeze)

Prelab

You will turn this in as part of the final report.

Procedural changes

We will not do part C of the lab. We will divide up the work for parts A and B and collect the class data.

Lab report

You will NOT write the memo described on page 6 of the experiment. Using the class data, you will complete all of the DATA TREATMENT and a subset of the QUESTIONS FOR CONSIDERATION.

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DATA: Use the last page of this document for the class data. (Do NOT use the table on page 10 of the lab!) This should be the first page of your report so put your names on this page. Use the data tables provided on page E34-5 of the lab handout for your individual data. This should be the second page of your report.

Comments on equations (1) and (2) on the first page of the lab handout

These equations are confusing, as they are written, because the definition of msolute is different in each equation. In equation (1), it refers to the total molality of particles. In equation (2), it refers to the molality of the ionic compound (calculated from the mass of NaCl or CaCl2 used). Make sure you keep this distinction straight when you use these equations. It would be nice if they used different symbols for the molality in each equation but they don’t. Below I use the symbol msolutefor the molality of the ionic compound and mparticles for the total molality of particles. For molecular compounds mparticles = msolute but for ionic compounds mparticlesmsolute.

Guidance for DATA TREATMENTand QUESTIONS FOR CONSIDERATION

Use the class averages for all calculations.

Part A

  1. Use the mass of the water (25.0 g) and the class average for the mass of each compound to calculate the actual molality of each solute (msolute). They will NOT be exactly 1.0 m!
  2. Skip this question (Tf is in the class data).
  3. Use equation (1)to calculate the total molality of particles (mparticles)
  4. Notice that mparticlesmsolute. Explain why this is true for ionic compounds but NOT for molecular compounds.
  5. The ideal value of i (the van’t Hoff factor) for ionic compounds is simply the number of ions that appear in solution each time a “formula unit” dissolves. A “formula unit” is just the empirical formula for the ionic compound (NaCl, CaCl2). In this experiment, you will see that the actual (experimental) value for i is lower than the ideal value.
  6. Remember that msolutei = mparticles. You calculated msolute and mparticlesabove so now you can calculate the actual (experimental) values for i. These should turn out to be lower than the ideal values determined in question 5.

Part B

  1. Tf values will be in the class data table. Use equation (1) and the average values for Tf to calculate the total molality of solute particles in all 5 solutions.
  2. Calculate the number of moles of solute in all 5 solutions. Do this by using the molality calculated in #1 and the mass of water (in kg) in each solution. Although ethylene glycol is the major component of anti-freeze, there are a number of other solutes present. Do NOT use the molar mass of ethylene glycol to calculate moles of solute.
  3. You just calculated these so there is nothing for you to do here!
  4. You will determine the experimental value of the “effective”molar mass by plotting the mass of antifreeze vs. moles of solute and adding a trendline through the data. (How will you determine the molar mass from the equation of the trendline?) The “effective” molar mass is a weighted average of all of the solutes in the antifreeze. Since antifreeze is mostly ethylene glycol, you should find that your experimental molar mass is close to that of ethylene glycol (C2H6O2). Your data consists of 5 masses and the corresponding number of moles. From this data, you could calculate 5 values for the molar mass and average them. (This is an “arithmetical” average.) However the molar mass determined from the graph (the “graphical” average) is probably more accurate. I will explain this at some point.
  5. The coldest temperature recorded at the base is given in the scenario on page 2 of the lab. You will convert molality to mass % because that is how the concentration of antifreeze is usually reported. (Actually, one usually uses volume % but that is very close to mass %.) You don’t tell the guy at the garage to fill your radiator with 2.5 molal antifreeze, you say “Give me 60% antifreeze”!

QUESTIONS FOR CONSIDERATION

  1. Ignore question 1 asked in the lab and answer the following question instead:

Due to ion pairing, the experimental values of the van’t Hoff factor for NaCl and CaCl2 are smaller than their ideal values. Why is the van’t Hoff factor for CaCl2further from its ideal value than is the value for NaCl? (If your data is not consistent with this description, pretend that it is when answering this question.)

Answer questions 2 and 3 but do NOT answer the other questions.

Organize your report in the following order:

  1. Class Data (put your names on this page)
  2. Individual Data (both your data and your partner’s data)
  3. Data Treatment (Show your work!)
  4. Excel file with spreadsheet and graph. (See my website for tips about formatting Excel files.)
  5. Questions for Consideration
  6. Prelab

If you feel that you need more space to do calculations or to answer questions, please do them on your own paper and attach them to the report. Remember that you can get up to 10% deducted from your report if it isn’t “pretty”. Don’t take a lot of time to accomplish this. Just write neatly, show your work clearly, rewrite errors rather than scratching them out, and organize the lab as instructed above. It is best to do the following. Each lab partner should write an individual report. Then get together with your partner to correct mistakes and produce the best single report to turn in.

Names: ______and ______

Experiment 34: CLASS DATA

(Use the averages for all calculations.)

Part A:

Group / Mass NaCl
(g) / Tf (oC) / Mass CaCl2
(g) / Tf (oC)
1
2
3
4
5
Average

Part B:

Mixture # / Mass Antifreeze (g) / Mass Water (g) / Tf (oC)
1
1
Average
2
2
Average
3
3
Average
4
4
Average
5
5
Average

1