Chemistry

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Physical Chemistry Lab Report Rubric 2012 (PDF, 2.62MB)

Chem. 122 General Chemistry Lab Discussion Examples

Experimental instructions

Lab 1 - Part 3. Calibrate your 5-mL Pipette Using a Balance

1. Obtain approximately 50 mL of deionized water (DI water).
2. Weigh a clean, dry 125-mL Erlenmeyer flask. Be sure to zero the balance before weighing.
3. Measure and record the temperature of the DI water.
4. Clean the 5-mL volumetric pipette with DI water. When clean, no water should be left on the inside walls of the pipette except for the last drop in the tip. The calibrated pipette is rated “to deliver” (TD) 5 mL, meaning the last drop of liquid does not drain from the tip.
5. Use the pipette to transfer exactly 5 mL of water into the weighed flask. Record the mass of the water and flask. Calculate the mass of the water.
6. Given the density of water at various temperatures (see Table 5), calculate the volume of water delivered from the pipette.
7. Repeat this measurement two more times. Calculate the average and the standard deviation in the volume delivered by your 5-mL pipette.

Discussion of Results Prompt: Total: 5 points

• 2-3 paragraphs summarizing results, written in 3^rd person, past-tense
• Does not include “procedure” information
• Draws conclusions from data & observations
• Results organized in summary tables (if needed)
• States known/theoretical values for any calculated values & compare to results
• Discuss systematic vs. random errors
• Does your Discussion answer the following:

1. What was calculated from this lab?
2. How was value acquired?
3. How does the experimental result compare to expected value(s)?
4. What error(s) would explain differences in (3)

Example 1 (Excerpt from part 3 of the experiment)

Score 2 / 5 points

In part three, the objective was to calibrate the pipette by weighing and measuring water in a flask. We found that the average volume of the water was 4.966 +/- 0.005 mL. The average mass of the water was 4.956 +/- 0.005 g. The results were as expected, as the pipette has an uncertainty of + 0.01 mL, from the table.

Other: Use of “we”, did not relate the results to the known value (5.00 mL pipette), did not need to report the mass of the water. 

Example 2 (Excerpt from part 3 of the experiment)

Score 3 / 5 points

In part three of the experiments we calibrated the 5mL pipette with a balance. This was done by measuring out approximately 5mL of deinozed water collected in the pipette to a beaker, getting the mass and density of the 5mL of water and then calculating the exact volume delivered by the pipette based off the known density of water at a given temperature. The point was to understand that there is a random error in the actual volume the pipette delivers. The average volume was 5.005mL and the average deviation was 0.018mL.

Other: Spelling and grammar mistakes, use of “we”, stated result but did not relate them to what the volume should be for the pipette. 

Example 3 (Excerpt from part 3 of the experiment)

Score 4.5 / 5 points

The objective of the third part of the lab was to learn how to calibrate a pipette by determining the true volume of the water with a balance. Through three trials of 5 mL, we found the true volume of the water to be 4.988 mL, 5.001 mL, and 4.989 mL, with an average of 4.993 mL and a deviation of 0.006 mL. With results tending to be lower than 5 mL, we can expect the pipette to be slightly miscalculated less than 5 mL. However, we can also claim such inaccuracies to human error, as the trials are all within the +/- 0.02 mL uncertainty of a 25 mL pipette. Also, the trials tend to be fairly precise, as the deviation of each result is well less than the 0.02 mL uncertainty.

Other: Some word usage and grammar issues, avoid the use of “human error” in this case “random error from the user” would be better since it is more exact and does not imply the student made mistakes.