2.2: Formative/Summative Questions

A. Formative Questions

1. Why does the formation of the meniscus differ in the plastic graduated cylinder compared to the glass graduated cylinder?

Answer: In a glass cylinder water molecules have an affinity for adhering to glass (cohesion.) Water molecules have an affinity for other water molecules in a plastic cylinder (plastic is non-polar.) In either case, you measure the volume of the liquid at the center of the liquid level.

2. What is the purpose of the first stop on the micropipette? The second stop?

Answer: First stop aspirates the sample. Second stop dispenses the volume. Check me here please

3. Is it best to measure your sample volume by looking at the measurements from above the sample, at eye level, or below your sample?

Answer: Eye level

4. Which graduated cylinder would be best to measure 9 mL of a sample: 10 mL or 100 mL? 

Answer: 10mL

5. Compare and contrast accuracy and precision.

Answer: Accuracy is closest to true value. Precision is degree of repeatability.

6. What is the specific gravity of water? Explain.

Answer: Specific gravity (SG) is the density of a liquid compared to the density of water. Density is mass (solute)/volume (solution.) The density of water is ~1g/1ml, or 1.000. Since there are two solutions being compared, specific gravity does not have units.

B. Summative Questions

1. How do plastic and graduated cylinders differ? Which is more precise? Accurate?

Answers: I'm seeing a lot of answers for this, depending on various factors. What do you all want to convey here?

2. What is the purpose of the micropipette? How do the volumes measured using a micropipette differ from those measured using a Pasteur pipette?

Answer: Micropipettes measure small volumes (1 µL to 100 µL .) Pasteur pipettes are disposable, measure larger volumes, and can be less accurate.

3. How do volumetric and Erlenmeyer flasks differ? Which is more precise? Accurate?

Answer: Volumetric flask (narrow neck) is more accurate and precise.

4. What is the equation used to calculate the molarity of a solution, if given the mass and volume of the solute?

Answer: Grams of solute = [molecular weight (grams)] x [volume (liters)] x [desired molarity].

5. What are the steps necessary to prepare a series of serial dilutions, where each dilution is 1/10 the concentration of the previous solution?

Answer: Add 1-part of the stock solution, to 9-parts water/solvent.

6. What is the purpose of a hand refractometer?

Answer: Measures the specific gravity of a solution.

7. What is specific gravity and how does s.g. vary as a solution increases in concentration?

Answer: A measure of the density of a solution over the density of water. As a solution increases in concentration (mass of solutes/mL solution,) the specific gravity increases.