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.
.
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 msolute for the molality of the ionic
compound and mparticles for the total molality of particles. For molecular compounds mparticles = msolute but
for ionic compounds mparticles > msolute.
Guidance for DATA TREATMENT and 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 mparticles > msolute. 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 mparticles above 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.
1
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 CaCl2 further 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.
2.
3.
4.
5.
6.
Class Data (put your names on this page)
Individual Data (both your data and your partner’s data)
Data Treatment (Show your work!)
Excel file with spreadsheet and graph. (See my website for tips about formatting Excel files.)
Questions for Consideration
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.
2
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
3