The Foundations of Chemistry

DETERMINATION OF MOLECULAR WEIGHTS AND

MOLECULAR FORMULAS OF GASEOUS SUBSTANCES

In Section 2-10 we distinguished between simplest and molecular formulas of compounds.

We showed how simplest formulas can be calculated from percent compositions of

compounds. The molecular weight must be known to determine the molecular formula

of a compound. For compounds that are gases at convenient temperatures and pressures,

the ideal gas law provides a basis for determining molecular weights.

EXAMPLE 12-12 Molecular Weight

A 0.109-gram sample of a pure gaseous compound occupies 112 mL at 100.°C and 750. torr.

What is the molecular weight of the compound?

Plan

We first use the ideal gas law, PVnRT, to find the number of moles of gas. Then, knowing

the mass of that number of moles of gas, we calculate the mass of one mole, the molecular

weight.

Solution

V0.112 L T100.°C273°373 K P750. torr0.987 atm

n0.00361 mol

The mass of 0.00361 mole of this gas is 0.109 g, so the mass of one mole is

30.2 g/mol

EXAMPLE 12-13 Molecular Weight

A 120.-mL flask contained 0.345 gram of a gaseous compound at 100.°C and 1.00 atm pres-

sure. What is the molecular weight of the compound?

Plan

We use the ideal gas law, PVnRT, to determine the number of moles of gas that filled

the flask. Then, knowing the mass of this number of moles, we can calculate the mass of

one mole.

Solution

V0.120 L P1.00 atm T100.°C273°373 K

n0.00392 mol

(1.00 atm)(0.120 L)




0.0821 m

L

o

a

l

t



m

K

(373 K)

PV



RT

0.109 g



0.00361 mol

_?_g



mol

(0.987 atm)(0.112 L)




0.0821m

L

o

a

l

t



m

K

(373 K)

PV



RT

1 atm



760. torr

12-10

The molecular weight of the gas is
30.2 amu. The gas could be ethane,
C 2 H 6 , MW30.1 amu. Can you
think of other possibilities? Could the
gas have been NO, CH 3 OH, O 2 , or
CH 3 NH 2?

454 CHAPTER 12: Gases and the Kinetic–Molecular Theory

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 12.6, Using Gas Laws:
Determining Molar Mass.