Molecularity: Fundamental Properties

1 lab period; work in pairs. Complete the Preparation page before laboratory.

Goals

• To explore the volume-mass relationships of various metal samples.
• To determine whether the physical form of a metal affects its mass-volume relationship.
• To determine consequences of mass-volume relationships.

Pure substances may be characterized by their physical and chemical properties. These properties may in fact be used to identify substances. In this experiment, we will develop an understanding of a simple and useful physical property of a pure substance by exploring the relationship between the masses and volumes of metal samples.

Mass-volume relationships of solids and liquids are measured using a device called a pycnometer, a glass bulb fitted with a stopper through which runs a capillary bore. By filling the inside volume of the pycnometer completely with a liquid, the mass of the liquid corresponding to that volume may be readily determined. To explore the mass-volume relationship of a solid substance requires a somewhat more subtle approach, as we will see.

In this experiment, you will use a precision balance and a pycnometer to explore mass-volume relations of two assigned metals, to answer the focus questions below. Before coming to lab, you should have thought a bit about, first, how to measure the volume of the pycnometer; and second, how to use the pycnometer to determine the volume of a sample of metal, knowing only that the sample will fit through the somewhat narrow mouth of the pycnometer.

As you proceed through the experiment, keep the following questions in mind. When you have finished the experiment, provide answers to them.

1. Why is the stopper of the pycnometer designed with a capillary bore?
2. Once you have determined the volume of the pycnometer, is it necessary that the inside of the pycnometer bulb be completely dry before you use it to determine the mass and volume of a metal sample? Explain your answer.
3. Why is it important to carry out your determinations in triplicate?
4. Pool your results with those of other students studying the same metal, and "make sense" of them (i.e., are there any regularities in the data?)
5. Is the mass-volume relationship of a metal dependent upon its bulk form?
6. Are mass-volume relationships of metals related to their atomic weights?

• Pasteur pipets/bulbs
• Aspirator trap
• analytical balance
• pycnometer
• metal samples (Cu, Zn, Al, Pb, Fe, Mg, Sn, Ni, Ti, Pt)

Eye protection must always be worn in the laboratory. The materials used in this experiment are not harmful; however, good lab practice requires the use of eye protection.

Record all data in your laboratory notebook. The instructor will assign you two metals to study, and will indicate the bulk form (granular, powder, wire, sheet, etc.) that you are to use.

Density Using a Pycnometer. Proper use of the analytical balance and the pycnometer will be demonstrated in lab. Here are some things to keep in mind when using your pycnometer.

• The pycnometer is delicate and expensive; exercise care when handling it.
• the bulb and stopper of the pycnometer are both engraved with the same number. Be sure that you do not inadvertently switch stoppers with someone else; check the numbers occasionally.
• The pycnometer must be clean and DRY before the initial weighing.
• To fill the pycnometer with liquid, use a Pasteur pipet to fill the bulb to about halfway up the neck (there is usually a white mark). Then slowly insert the capillary stopper.
• When full, there should be NO air bubbles in the bulb or capillary of the pycnometer, and no air space at the top of the capillary.
• Before weighing the full pycnometer, the outside should be perfectly dry.

With your lab partner, try to come up with a step-by-step procedure for using distilled water to determine precisely the volume of the pycnometer. Ask the instructor to review your procedure. Then carry out the procedure in triplicate to determine the volume of your pycnometer as accurately as possible. Dry the pycnometer thoroughly by aspiration between runs.

With your lab partner, try to come up with a step-by-step procedure for determining the mass and volume of a sample of metal using the pycnometer. Ask the instructor to review your procedure. Then carry out the procedure in triplicate to determine the mass and volume of a sample of your first assigned metal. Please do NOT throw away the metal samples--they can be recycled for use by other students. Repeat for your second assigned metal. Be sure to clean and dry the pycnometer in between runs.

Clean-up. When you have finished all of your work:

• Clean all glassware by recommended procedures, shake off excess water, and place in the drying oven.
• Clean the pycnometer with distilled water, and dry by aspiration.
• Return all borrowed equipment to the instructor before leaving lab.
• Return used metal samples as directed by the instructor.
• Clean up your work area before leaving lab.

No disposal required.

Preparation Questions.