Cracking the SAT Chemistry Subject Test

The molecules are moving, banging and bumping against one another and
against the walls of the box in every which way. Because they bang into the
walls of the box, they create pressure against the walls. When we talk about
pressure, in relation to gases, we’re talking about the amount of force the gas
particles are exerting on the walls of their container, per unit area of the
container. As we’ve said before, we measure pressure in torr or in millimeters of
mercury (mmHg) or atmospheres (atm). Each of these units represents a unit of
force per area, and 760 torr = 760 mmHg = 1 atm.

Ideal   Gas Behavior

Under   conditions  of  low

pressure,   gas molecules

occupy  very    little  volume

relative    to  the volume  of

the container.  Because

ideal   gases   have    zero

volumes,    gases   under   low

pressure    conform more

closely to  the ideal   gas

law than    gases   under

higher  pressure.   They

act similarly   at  higher

temperatures,   and forces

between gas molecules

are negligible. Because

ideal   gases   do  not exert

any forces  on  each    other,

gases   at  high    temperatures

behave  more    ideally

than    gases   under   cooler

temperatures.

All of the gases you’ll see on the SAT Chemistry Subject Test are assumed to be
ideal gases.

Ideal   Gas Assumptions

Molecules   of  an  ideal   gas do  not attract or  repel   each    other.

Molecules   of  an  ideal   gas occupy  zero    volume.

No gas ever acts completely as an ideal gas. In real gases, molecules attract each
other slightly, which causes them to strike the walls of their container with