Gases

Part 1

Elements that exist as gases at 250C and 1 atmosphere

• Gases assume the volume and shape of their containers.

• Gases are the most compressible state of matter.

• Gases will mix evenly and completely when confined to the same container.

• Gases have much lower densities than liquids and solids.

Physical Characteristics of Gases

Units of Pressure

1 pascal (Pa) = 1 N/m2

1 atm = 760 mm Hg = 760 torr

= 101,325 Pa = 14.7 psi = 29.92 in. Hg

Barometer

Pressure = ForceArea

(force = mass x acceleration)

Sea level 1 atm

4 miles 0.5 atm

10 miles 0.2 atm

Boyle’s LawBoyle’s LawP P αα 1/V 1/VThis means Pressure and This means Pressure and

Volume are INVERSELY Volume are INVERSELY PROPORTIONAL if moles PROPORTIONAL if moles and temperature are and temperature are constant (do not change). constant (do not change). For example, P goes up as For example, P goes up as V goes down.V goes down.

PP11VV11 = P = P22 V V22

Robert Boyle Robert Boyle (1627-1691). (1627-1691). Son of Earl of Son of Earl of Cork, Ireland.Cork, Ireland.

Charles’s Charles’s LawLaw

If n and P are constant, If n and P are constant,

then V then V αα T TV and T are directly V and T are directly

proportional.proportional.VV11 V V22

==

TT11 T T22

• If temperature goes up, the If temperature goes up, the

volume goes up!volume goes up!

Jacques Charles Jacques Charles (1746-1823)(1746-1823)

Gay-Lussac’s LawGay-Lussac’s LawIf n and V are If n and V are

constant, constant, then P then P αα T T

P and T are directly P and T are directly proportional.proportional.

PP11 P P22

==

TT11 T T22

If temperature goes up, the If temperature goes up, the

pressure goes up!pressure goes up!

Joseph Louis Gay-Joseph Louis Gay-Lussac (1778-1850)Lussac (1778-1850)

Combined Gas Law• The good news is that you don’t have to

remember all three gas laws! Since they are all related to each other, we can combine them into a single equation. BE SURE YOU KNOW THIS EQUATION!

P1 V1 P2 V2

=

T1 T2

STP in chemistry stands for Standard Temperature and

PressureStandard Pressure =

1 atm (or an equivalent)

Standard Temperature = 0 deg

C (273 K)

STP allows us to compare amounts of

gases between different pressures and temperatures

STP allows us to compare amounts of

gases between different pressures and temperatures

Avogadro’s Law

V number of moles (n)

V = constant x n

V1/n1 = V2/n2

Constant temperatureConstant pressure

Ideal Gas Equation

Charles’ law: V T(at constant n and P)

Avogadro’s law: V n(at constant P and T)

Boyle’s law: V (at constant n and T)1P

V nT

P

V = constant x = RnT

P

nT

PR is the gas constant

PV = nRT

The conditions 0 0C and 1 atm are called standard temperature and pressure (STP).

PV = nRT

R = PVnT

=(1 atm)(22.414L)

(1 mol)(273.15 K)

R = 0.082057 L • atm / (mol • K)Experiments show that at STP, 1 mole of an ideal gas occupies 22.414 L.

Density (d) Calculations

d = mV =

PMRT

m is the mass of the gas in g

M is the molar mass of the gas

Molar Mass (M ) of a Gaseous Substance

dRTP

M = d is the density of the gas in g/L

A 2.10-L vessel contains 4.65 g of a gas at 1.00 atm and 27.00C. What is the molar mass of the gas?

dRTP

M =