Molecular
Composition
of
Gases
Volume – Mass Relationship
Gay-Lussac: Volume can be
expressed as ratios of whole
numbers.
2H2(g) + O2(g) 2 H2O(g)
2 vol 1 vol 2 vol
Dalton: Atoms are single indivisible
particles.
H + ½O HO1/2
Avogadro’s Law
Equal volumes of gases contain
Equal numbers of molecules
V = kn
2H2(g) + O2(g) 2H2O(g)
2 molecules 1 molecules 2 molecules
2 moles 1 moles 2 moles
2 vols 1 vols 1 vols
Separated At Birth?
Standard Molar Volume
At Standard
Temperature and
Pressure the
volume of gas that
contains one mole
of gas
22.4 L = 1
mole @ 1 atm &
273K
Ideal Gas Law
Mathematically relates volume to
– Pressure V α 1/P (Boyle)
– Temperature V α T (Charles)
– Moles V α n (Avogadro)
V α (1/P) (T) (n)
V = (R Tn)/P
PV = nRT
R = 0.0821 (L · atm) / (mol·K)
Density / Molar Mass
n= m/M m = mass of gas
M = MW or molar mass
PV = (m/M)RT
PM = (m/V)RT (rearrange eqn)
M = mRT/PV (solve for Molar mass)
Density = D = m/V
PM = DRT
D = PM/RT (to solve for density)
Diffusion / Effusion
Effusion
Effusion can be used to estimate molar
mass.
Rates of effusion & diffusion depend on
relative velocities of gas molecules.
V α 1/m
Lighter molecules have higher V than
heavier molecules at same T.
Graham’s Law
KE = ½ mV2
For 2 different gases at same T
½ MAV A2 = ½ MBV B2
(V A2 )/ (V B2 ) = MB /MA
(Rate A)/(Rate B) = (MB/MA)1/2
Also = (DB/DA)1/2