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Home Concept Builders Gases and Gas Laws Pressure Concepts Help Help 8

Pressure Concepts - Questions 8 Help

There are a variety of units by which the pressure of a gas sample can he expressed. Atmospheres (abbreviated as atm), pounds per square inch (abbreviated as psi), and millimeters of mercury (abbreviated as mm Hg) are three such units. The following unit equivalencies provide a feel for the relative size of each unit:

1 atm = 14.7 psi = 760 mm Hg

There are three similar versions of this question. Here is one of those versions:
 

Version 1:

The pressure of three different samples of gas is shown below. The pressure value is expressed in different units. Rank the three samples according to their pressure.
 

In this question, you are given three pressures ... in three different units and you have to compare them to determine the greatest, middlest, and smallest pressure value.
 

Get a Game Plan

Like any problem in Chemistry, it is important to have a Game Plan. A good game plan equips you with a strategy for proceeding from the given information in the problem to the destination - the answer to the problem.

There are probably two basic game plans that a student could use to answer this question. The first Game Plan is to treat the question like a conversion problem. Select a unit - like atmospheres - and convert the two non-atm amounts to atmospheres. That is convert the pressure value in psi to atm and the pressure value in mm Hg to atm. Using this Game Plan, you would have to use the unit equivalencies (1 atm = 14.7 psi AND 1 atm = 760 mm Hg) to make two conversion factors and convert to Pressure in atm units. Once all three Pressure amounts are in atm units, you can easily compare to determine which is greatest, middlest, and smallest.

The second Game Plan (the one we are strong advocates of) is much more conceptual in nature. It involves head math and some thinking. This Game Plan begins by conceptualizing the size of an atm to that of a psi to that of a mm Hg. Since approximately 15 psi units are equivalent to 1 atm unit, a single psi unit must be rather small compared to a single atm unit. In fact, a psi is roughly 1/15-th of an atmosphere. Similarly, a mm Hg unit is smaller still - about 1/760-th of an atm. So when you use this thinking you realize that it will take 760 mm Hg to be equal to 1 atm. In this question you have 1 atm but only one-half of the required 760 mm Hg to match it. So the 1 atm is greater than the 380 mm Hg. And in this question you would need about 15 psi to match the 1 atm and you have twice what is needed since the "psi pressure" value is 30 psi. So you can see how a little conceptualizing of the units and some simple head math goes a long way towards getting this answer correct without using a calculator.
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