Science Buddies: "Ask an Expert"

Hi,
My experiment data is different than the expected, calculated data. My experiment average temperature depression for salt is .67 (2.9g), 2.5(5.8g), 3.67(11.7g). For sucrose it’s .67 (17.1g), 1.67 (34.2g) and 5.67 (68.5g). Average freezing point for water was 0.

The calculated results are 1.86 (2.9g), 3.68(5.8g), 7.44(11.7g) for salt and .73 (17.1g), 1.86 (34.2g) and 3.72 (68.5g) for sucrose .
I don’t understand why the temperature depression for sucrose in my experiment is greater than salt when the calculated results show the opposite.

Thank you

Hi msantangelo,

As I look at your data, a couple of thoughts immediately come to mind.

The first thought regards your data table and calculations. How precise were your temperature measurements? Did you always measure to 0.1 degrees? Or to 0.5 degrees? Or just 1 degree accuracy? In other words, is the number "-2” in column D of your spreadsheet just -2 degrees? Or is it -2.0 degrees? It doesn't look like you recorded temperature measurements to two decimal places (e.g., -2.00 degrees), so your average measured freezing point depression and calculated freezing point depression should be rounded to the first decimal place (if you were measuring to 0.1 degree precision).

The second deals with your unexpected results. I checked your calculations, and they are correct. So, the problem isn't that your calculations are incorrect. If you graph your data with molality on the x-axis and your measured deltaT on the y axis, you should see a straight line for both the NaCl and sucrose data sets. If your data were consistently off by some factor from the calculated freezing point depression, then the problem is probably related to the accuracy of your thermometer. But, this doesn't seem to be the case for your data. With your data the NaCl data curve over to the right while the sucrose data curve up toward the top of the graphs. What this tells me is that for NaCl, you were under-measuring the deltaT while for sucrose you were over-measuring deltaT as molality increased. If you look at your data for 1 molal solutions, you see that the NaCl had a greater freezing point depression than sucrose. I would try re-making the 2 molal solutions and re-running those three tests. Make sure that the NaCl and sucrose are completely dissolved. Post back after you re-doing those 2 molal solution experiments, and let me know how it turns out.

I did not use a digital thermometer so the measurements were just 1 degree accuracy. The one case in my data where I listed 2.5 was because it was clear the reading was in between 2 and 3. Maybe I should change this to 3? I will re-do my experiment for test liquids # 3 and #6 and let you know the results. Thank you

Hi msantangelo,

I look forward to hearing about your new results! In terms of the temperature measurements, if your thermometer is marked for every degree (i.e., it is marked 1 degree, 2 degrees, 3 degrees, etc.), you can report your temperatures to the 0.1 degree level. You are on the right track with the 2.5 degree measurement. For example, let's say that you measure a temperature that isn't exactly 2 degrees, but isn't high enough in the thermometer to be 3 degrees. What you do is estimate how far up the thermometer liquid is between the 2 and the 3. Is it 10% up? Then the measurement is 2.1 degrees. If it were 90% of the way up, it would be 2.9 degrees, etc.