•Gas pressure is due to the force of gaseous particles colliding with their container.

•Torricelli invented the barometer to measure atmospheric pressure.

•Units used: mmHg, atm, kPa, psi

•Using a manometer: gas pressure > air pressure Pgas = Patm + h gas pressure < air pressure Pgas = Patm – h

•Describe the relationship between pressure and volume of a gas.

•Describe the relationship between temperature and volume of a gas at a constant pressure.

•Define absolute zero.

•Solve problems for both Boyle’s and Charles’ Law.

Pre

ssur

e (k

Pa)

Volume (mL)

BOYLE’S LAW – Pressure vs. Volume

BOYLE’S LAW:

The volume of a given amount of gas (held at a constant temperature) varies inversely with the pressure.

P α 1 V

What will be the volume of 40 L of gas (held at a constant temperature) if the pressure is doubled? Reduced to 1/3rd ?

P α 1 V

1/3 α 1 1/3

2 α 1 2

1 = 3 1/3 120 L

20 L

PV = k

=

P1V1P2V2

P α 1 V

If 3 L of gas is initially at a pressure of 1 atm, what would be the new pressure required for a volume of 0.5 L?

6 atm of pressure will change 3 L of a gas at 1 atm to 0.5 L.

=

P1V1 P2V2

=

(1)(3) P2 (0.5)

A syringe contains 20 mL of a gas at 100 kPa. The pressure in the syringe is changed to 25 kPa. What is the new volume of the gas?

The gas will expand to 80 mL.

=

P1V1 P2V2

=

(100)(20) V2(25)

Absolute Zero

Jacques Charles (1746-1823) • Volume of gas changed with only 1oC change in temperature.

An increase of 273°C, doubled the volume.

V α T

CHARLES’S LAW: Volume of a given gas (held at a constant pressure) varies directly with the temperature

Temperature (oC)

Vol

ume

(mL

)

CHARLES’S LAW – Temp vs. Volume

William Thomson (1824-1907) • Created Kelvin Scale where –273°C is the lowest temperature possible or Absolute Zero.

“Hypothetical” – called an ideal gas model:• predicts that with zero kinetic energy comes zero volume.

TK = TC + 273 TC = TK - 273

Stated as “315 Kelvin”- no degrees.

Ideal Gas Model:• engaged in random motion (normal)• obey conservation of energy (normal)• non-interacting particles

(IMFs alter results)

Most real gases behave like “ideal gases,” butformula fails at lower temperatures OR higher pressures.

=

V1 V2

T2

T1 **All temperatures must be in Kelvin.

V α T V = k T

What is the new volume of a gas if 100 mL of the gas at 25°C is cooled to –25°C?

25°C + 273 = 298 K–25°C + 273 = 248 K

The new volume is 83.2 mL.

=

V1 V2

T2

T1

=

100 V2 -25 25

=

100 83.2 mL (248) 298

If the volume of a gas at –73°C is doubled to 48.0 L, calculate the final temperature in degrees Celsius.

400 K – 273 = 127°C

–73°C + 273 = 200 K

=

V1 V2

T2

T1

=

24.0

48.0 400 K (200)

•Boyle described the inverse relationship between the pressure and volume of a gas at a constant temperature.

•Charles described the direct relationship between the temperature and volume of a gas at a constant pressure.

•William Thomson (Lord Kelvin) developed the Kelvin temperature scale.

•At zero Kelvin (absolute zero) , the volume of a gas will theoretically be zero.

•When solving gas problems the temperature must always be converted to Kelvin.