John Hutchinson - General Chemistry II

By: John Hutchinson

The individual molecules of differentcompounds have characteristic properties, such as mass, structure,geometry, bond lengths, bond angles, polarity, diamagnetism orparamagnetism. We have not yet considered the properties of massquantities of matter, such as density, phase (solid, liquid or gas)at room temperature, boiling and melting points, reactivity, and soforth. These are properties which are not exhibited by individualmolecules. It makes no sense to ask what the boiling point of onemolecule is, nor does an individual molecule exist as a gas, solid,or liquid. However, we do expect that these material or bulkproperties are related to the properties of the individualmolecules. Our ultimate goal is to relate the properties of theatoms and molecules to the properties of the materials which theycomprise.

Achieving this goal will require considerableanalysis. In this Concept Development Study, we begin at a somewhatmore fundamental level, with our goal to know more about the natureof gases, liquids and solids. We need to study the relationshipsbetween the physical properties of materials, such as density andtemperature. We begin our study by examining these properties ingases.

1.3. Observation 1: Pressure-Volume Measurements on Gases

It is an elementary observation that air has a"spring" to it: if you squeeze a balloon, the balloon rebounds toits original shape. As you pump air into a bicycle tire, the airpushes back against the piston of the pump. Furthermore, thisresistance of the air against the piston clearly increases as thepiston is pushed farther in. The "spring" of the air ismeasured as a pressure, where the pressure P isdefined

(1.1)

F is theforce exerted by the air on the surface of the piston head and A is thesurface area of the piston head.

For our purposes, a simple pressure gauge issufficient. We trap a small quantity of air in a syringe (a pistoninside a cylinder) connected to the pressure gauge, and measureboth the volume of air trapped inside the syringe and the pressurereading on the gauge. In one such sample measurement, we might findthat, at atmospheric pressure (760 torr), the volume of gas trappedinside the syringe is 29.0 ml. We then compress the syringeslightly, so that the volume is now 23.0 ml. We feel the increasedspring of the air, and this is registered on the gauge as anincrease in pressure to 960 torr. It is simple to make manymeasurements in this manner. A sample set of data appears in.

Table 1.1. Sample Data from Pressure-Volume Measurement

Pressure (torr)	Volume (ml)
760	29.0
960	23.0
1160	19.0
1360	16.2
1500	14.7
1650	13.3

Figure 1.1. Measurements on Spring of the Air

An initial question is whether there is aquantitative relationship between the pressure measurements and thevolume measurements. To explore this possibility, we try to plotthe data in such a way that both quantities increase together. Thiscan be accomplished by plotting the pressure versus the inverse ofthe volume, rather than versus the volume. The data are given in.