CHEM 212 Laboratory



CHEM 212 Laboratory

Determination of Molecular Mass by Freezing Point Depression

Data Analysis Supplement

Reference: General Chemistry Laboratory Experiments, Suzanne W Slayden, 1999, 3rd edition, Pages 107-111

Data: After you have noted down the time/temperature readings for pure water, initial and final buret readings for isopropyl alcohol, and time/temperature readings for mixture of isopropyl alcohol and water, enter your own data into the appropriate Web-based data entry form, using a computer in the lab. In your measurements and calculations, remember to pay attention to uncertainties and significant figures.

Results: Outside of class, retrieve your data in spreadsheet form (along with that of the entire class) from: . Then use Excel to calculate mass of water, mass of isopropyl alcohol, freezing point depression, molality, moles of isopropyl alcohol in mixture, molecular mass of isopropyl alcohol and percent error.

Your lab report should include the following in addition to the pages from your lab notebook:

• An Excel sheet “Raw Data” showing the raw data for the entire class (as downloaded). Make sure that you format it so that units are included. A typical example of the raw data sheet is shown below:

Raw Data Sheet

| |A |B |C |D |E |

|1 |ID |V_water |T_water |V_alcohol |T_alcohol |

| | |Ml |(C |ml |(C |

| | | | | | |

|2 |1 |25 |0 |6.33 |-10.1 |

|3 |2 |25 |0 |6.4 |-9.85 |

Explanation:

• V_water = Volume of water

• T_water = Temperature of pure water

• V_alcohol = Volume of Isopropyl alcohol

• T_alcohol = Temperature of Isopropyl alcohol and water

• A 2nd Excel worksheet “Results” showing the calculated results for each student in the class – each in a separate row. Make sure to show an example calculation for each column. Each table should be formatted according to the instructions given in the Excel Tutorial on the retrieval page.

A typical example of the results sheet is shown below:

Results Sheet

|A |B |C |D |E |F |G |H | |1 |ID |m_H2O

kg |m_Iso

g |dt_f

°C |m_expt

m |Mol

(Isopropyl) |MW

(Isopropyl) |% Error | | | | | | | | | | | |2 |1 |0.0250 |8.0636 |10.10 |5.430 |0.1359 |59.2947 |1.31 | |3 |2 |0.0250 |8.1528 |9.85 |5.295 |0.1326 |61.4720 |2.30 | |

Explanation:

• m_H2O = Mass of distilled water solvent

• m_Iso = Mass of Isopropyl alcohol

• dt_f = Freezing point depression

• m_expt = Molality

• Mol (Isopropyl) = Moles of isopropyl alcohol in mixture

• MW (Isopropyl) = Molecular mass of isopropyl alcohol

• % Error = Percent error

• An Excel chart ‘Freezing Point Determination’ with the variables time on the x-axis and temperature on the y-axis to determine the freezing point.

• A 2nd Excel chart ‘ Histogram’ showing a histogram of the distribution of percent error in the experimental molecular masses of isopropyl alcohol. Use equally spaced “bins” chosen to display the “spread” in the class results. Refer to the histogram tutorial on the website.

Discussion: In your discussion section address the following questions:

• Define the freezing point of a solution?

• What may be a reason due to which an alcohol/water mixture does not freeze at a constant temperature?

• What are the possible sources of error in this experiment? Refer in your answer to the class histogram as well as to your own results.

• Discuss the shape of the histogram. What does it tell you about the nature of the error in the class’ measurements?

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