The Foundations of Chemistry

V101 L

You should now work Exercise 46.

You may wonder why pressures are given in torr or mm Hg and temperatures in °C.

This is because pressures are often measured with mercury barometers, and temperatures

are measured with Celsius thermometers.

EXAMPLE 12-10 Ideal Gas Equation

A helium-filled weather balloon has a volume of 7240 cubic feet. How many grams of helium

would be required to inflate this balloon to a pressure of 745 torr at 21°C? (1 ft^3 28.3 L)

Plan

We use the ideal gas equation to find n,the number of moles required, and then convert to

grams. We must convert each quantity to one of the units stated for R. (R0.0821

Latm/molK)

Solution

P745 torr0.980 atm T21°C273°294 K

V7240 ft^3 2.05 105 L n_?_

Solving PVnRTfor nand substituting gives

n8.32 103 mol He

_?_g He(8.32 103 mol He)
4.00  3.33 104 g He

You should now work Exercise 47.

g



mol

(0.980 atm)(2.05 105 L)




0.0821m

L

o

a

l

t



m

K

(294 K)

PV



RT

28.3 L



1 ft^3

1 atm



760 torr

(4.00 mol)
0.0821

m

L

o

a

l

t



m

K

(303 K)



0.984 atm

nRT



P

A helium-filled weather balloon.

452 CHAPTER 12: Gases and the Kinetic–Molecular Theory

Problem-Solving Tip:Watch Out for Units in Ideal Gas Law

Calculations

The units of Rthat are appropriate for ideal gas law calculations are those that involve

units of volume, pressure, moles, and temperature. When you use the value R0.0821

Latm/molK, remember to express all quantities in a calculation in these units. Pres-

sures should be expressed in atmospheres, volumes in liters, temperature in kelvins, and

amount of gas in moles. In Examples 12-9 and 12-10 we converted pressures from torr

to atm. In Example 12-10 the volume was converted from ft^3 to L.