# 3 Demonstrate the Ability to Use Dimensional Analysis to Convert Pressure Units

CH 11 NOTES

**The properties of gases that are easily observed are the relationships among pressure, volume, temperature and mass. Gas Laws were developed based upon these observations.**

Objectives:

#1 ~ Model the effects of changing number of particles, mass, temperature, pressure, and volume on a gas using kinetic theory.

#2 ~ Evaluate atmospheric pressure.

#3 ~ Demonstrate the ability to use dimensional analysis to convert pressure units.

Review: What is gas pressure?

The force per unit area that the particles in the gas exert on the walls of their container.

Section 11.1

What three things that affect the pressure of a gas?

1.# of particles/(mass):the more often gas particles collide with the walls of their container, the greater the pressure.

2.temperature:higher temperature means higher kinetic energy; particles move faster and collide with the walls of the container more often and with greater force, therefore the pressure is greater. * If the volume of the container and the number of particles are not changed, the pressure of a gas increases in direct proportion to the kelvin temperature.

3.volume:when volume is not held constant, and temperature increases, volume increases at constant pressure. If volume decreases, pressure of a gas, increases.

So, how does increasing any of these three things change the pressure?

# of particlespressure

temperaturepressure

volumepressure

**What happens to volume if the # of particles (n) is increased and temperature and pressure stay the same?

n increases, then V increases (T and P are constant)

**What happens to volume if temperature is increased and n and P stay the same?

T increases, then V increases (n and P are constant)

Devices to Measure Pressure

Barometer : instrument that measures the pressure exerted by the gases of the atmosphere.

*Highest pressure occurs at lowest altitude and vice versa

What units do we measure pressure in?

Standard atmosphere (atm): the pressure that supports a 760-mm column of mercury in a barometer; the pressure exerted by the gases of the atmosphere at sea level

1.00 atm = 760 mm Hg

**When you hear a meteorologist say, "the barometer is falling", it means that the column of mercury is falling because atmospheric pressure is decreasing**

Pressure Gauge: instrument that can measure the gas pressure on the inside of a tire or tank

** This pressure is above atmospheric pressure**

If required to calculate absolute pressure, you must add the pressure from the gauge to the atmospheric pressure.

Pressure Units

SI Unit for pressure = the pascal (Pa); for convenience we use the kilopascal. (How many pascals is this?______)

- One atm = 101.3 kPa (this is the pressure exerted by the atmosphere on one square meter, m2, of the earth)

- One atm = 14.7 psi (pounds per square inch; this is the pressure exerted by the atmosphere on one square inch, in2, of the earth)
- 101.3kPa = 1.00 atm = 14.7 psi = 760 mm Hg; these are all equivalent pressures!

Conversions

*Dimensional Analysis/Factor Label Method *: a way of changing the units of a measurement without affecting its value; uses conversion factors equal to one.

Practice Problems, p. 379

PEA~C Story

Write a short story about a molecule of "air" confined in the tire of an automobile. The events in your story must relate to the kinetic model of gases.These events/experiences of the molecule consist of what occurs whenthe car travels from the northern part of the country, where temperatures are below freezing, to the south where temperatures are in the eighties. Include what happens to the molecules when tire pressure is measured, when air is added, and what the molecule experiences in the event of a blowout.

P ~ State your point/purpose

E ~ Cite evidence

A ~ Show how evidence supports your point/purpose

C ~ Conclusion

DEMO:Gas against Fire

** If you were having a balloon throwing contest, which one of the following balloons would travel the furthest (horizontally)? Shortest? Why?

Helium, Carbon Dioxide, Air, Oxygen

DEMO:Why doesn't water pour out?

DEMO:Can air pressure do that?

DEMO: Who likes to talk?

Section 11.2

Objectives:

#1 ~ Analyze data that relate temperature, pressure, and volume of a gas.

#2 ~ Model Boyle’s Law and Charles’s Law using kinetic theory.

#3 ~ Predict the effect of changes in pressure and temperature on the volume of a gas.

#4 ~ Relate how volumes of gases react in terms of the kinetic theory of gases.

The Gas Laws

*Boyle's Law : Relates pressure and volume of a gas*

DEMO:The Diving Dropper

What is happening?

- When the bottle is squeezed, the air in the dropper is compressed OR we can say that the pressure on the air in the dropper has increased.
- Water fills the space in the dropper OR we can say that the volume of air has decreased, making room for the water.
- When the water fills the dropper, the dropper has more mass, and it “dives”.
- How does it surface?

Boyle's Law : states that at constant temperature the pressure and the volume of a gas are inversely proportional…stated more simply:

Pressure↑Volume↓

Volume↑Pressure↓

**What should the graph of pressure (x-axis) vs. volume (y-axis) look like based on this law?

DEMO: How does breathing illustrate Boyle's Law?

- Inhaling increases the volume available in the lungs, and decreases the pressure in the lungs. This allows the greater atmospheric pressure to force air in.
- Exhaling decreases the volume in the lungs, and therefore increases the pressure in the lungs. This increase in pressure allows us to expel the air from our lungs.

DEMO: How do you blow up and shrink a marshmallow?

- Write out the procedure for your demonstration in your journal. Please be prepared to share.
- Write out a detailed explanation of how this demonstration relates to Boyle’s Law.

**Explain how a weather balloon illustrates Boyle's Law, pp. 386-387

Practice Problems, p. 385

Charles's Law : Temperature and Volume

The volume of a gas at constant pressure is directly proportional to its Kelvin temperature

Temperature↑Volume ↑

Temperature↓Volume↓

Mathematical Formula:V1 = V2

T1 T2

**What should the graph of temperature (x-axis) vs. volume (y-axis) look like based on this law?

**Keep in mind that temperature MUST be in Kelvin!

DEMO: Charles’s Law

DEMO: How does an egg get in there?

Practice Problems, p. 391

Combined Gas Law : A combination of Boyle's and

Charles's Gas Laws

Mathematical Formula:P1V1 = P2V2

T1 T2

Standard Temperature and Pressure (STP) : 0.00 0C or

273K and 1.00 atm

Practice Problems, p. 394

The Law of Combining Gas Volumes : At constant

temperature and pressure, the volumes of

gases combine or decompose in ratios of small,

whole numbers (never decimals!)

See Figure 11.15 for an example

Why? Avogadro's Principle : At constant T and P, the volume of a gas depends on the # of particles or, equal volumes of gases have equal # of particles at constant T and P