Chemistry Reference
In-Depth Information
Number
of moles
of gas
Pressure,
volume, and
temperature
Percent
composition
PV
nRT
See
Sec
tion
7.4
Mass
Empirical
formula
Molar
mass
See
Sec
tion
7.5
Molecular
formula
The molar mass is the mass divided by the number of moles:
0.507 g
0.0194
5
mol
78.1 g/mol
The mass of a mole of empirical formula units is
12.0 g
1.0 g
13.0 g
26.1 g
13.0 g
6
so the molecular formula is twice the empirical formula,
C
2
H
2
.
Practice Problem 12.23
A 3.53-g sample of a gaseous hydrocarbon
occupies 2.90 L at and 749 torr. The gas is composed of 79.89% C and
the rest hydrogen. What is the molecular formula of the gas?
24°C
We can calculate the density of a gas using the ideal gas law plus molar
mass data.
EXAMPLE 12.24
Determine the density of nitrogen at STP.
Solution
It is easy to calculate the number of moles of nitrogen gas per liter using the
ideal gas law:
n
P
RT
1.00 atm
(0.0821 L
#
atm/mol
#
K) (273 K)
V
0.0446
2
mol/L
