The Foundations of Chemistry

Solution

V 1 12.0 L P 1 80.0 kPa T 1 240.°C273°513 K

V 2 15.0 L P 2 107 kPa T 2 _?_

We solve the combined gas law equation for T 2.

 so T 2 858 K

K°C273° so °C858 K
273° 585°C

You should now work Exercises 32 and 33.

(107 kPa)(15.0 L)(513 K)



(80.0 kPa)(12.0 L)

P 2 V 2 T 1



P 1 V 1

P 2 V 2



T 2

P 1 V 1



T 1

448 CHAPTER 12: Gases and the Kinetic–Molecular Theory

Problem-Solving Tip:Units in Combined Gas Law Calculations

The combined gas law equation is derived by combining Boyle’s and Charles’s Laws, so

the comments in earlier Problem-Solving Tips also apply to this equation. Remember

to express all temperatures in kelvins. Volumes can be expressed in any units as long as

both are in the same units. Similarly, any pressure units can be used, so long as both are

in the same units. Example 12-4 uses torr for both pressures; Example 12-5 uses kPa for

both pressures.

AVOGADRO’S LAW AND THE STANDARD MOLAR

VOLUME

In 1811, Amedeo Avogadro postulated that

at the same temperature and pressure, equal volumes of all gases contain the same

number of molecules.

Many experiments have demonstrated that Avogadro’s hypothesis is accurate to within

about 2%, and the statement is now known as Avogadro’s Law.

Avogadro’s Law can also be stated as follows.

At constant temperature and pressure, the volume, V,occupied by a gas sample is

directly proportional to the number of moles, n,of gas.

V n or Vkn or k (constant P, T)

For two samples of gas at the same temperature and pressure, the relation between volumes

and numbers of moles can be represented as

 (constant T, P)

V 2



n 2

V 1



n 1

V



n

12-8

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 12.3, Gas Laws.