Cliffs AP Chemistry, 3rd Edition

The Free Style

This method is the one most commonly used, although in my opinion, it is the method of last

resort. Free style often results in aimless, rambling, messy, incomplete answers. This method is

simply writing paragraphs to explain the question. If you do adopt this method for an answer

(and many questions lend themselves only to this method), you must organize the paragraphs

before writing. Also review your list of key terms to see if there are any concepts you want to

add to your answers. Note, however, that adding thoughts at a later time is difficult with this

approach because they will be out of logical sequence. (Unlike the bullet and outline formats,

free style doesn’t leave you room to add more ideas where they belong.)

Essay 4

If one completely vaporizes a measured amount of a volatile liquid, the molecular weight of the

liquid can be determined by measuring the volume, temperature, and pressure of the resulting

gas. When one uses this procedure, one uses the ideal gas equation and assumes that the gas be-

haves ideally. However, if the sample is slightly above the boiling point of the liquid, the gas

deviates from ideal behavior. Explain the postulates of the ideal gas equation, and explain why,

when measurements are taken just above the boiling point, the calculated molecular weight of a

liquid deviates from the true value.

Answer

4. Restatement: Explain ideal gas equation and why MW measurements taken just above
   boiling point deviate.

The ideal gas equation, PV =nRT stems from three relationships known to be true for

gases under ordinary conditions:

1. The volume is directly proportional to the amount, V~ n


2. The volume is directly proportional to the absolute temperature, V~ T


3. The volume is inversely proportional to the pressure, V~ 1/P

We obtain n, the symbol used for the moles of gas, by dividing the mass of the gas by the mole-

cular weight. In effect, n =mass/molecular weight (n =m/MW). Substituting this relationship

into the ideal gas law gives

PV=mRTMW

Solving the equation for molecular weight yields

MW=mRTPV

Part I: Introduction

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