Everything Science Grade 11

5.6 CHAPTER 5. THERMAL PROPERTIES AND IDEAL GASES

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5.6 The ideal gas equation ESBBD

In the early 1800’s, Amedeo Avogadro hypothesised that if you have samples of different gases, of the

same volume, at a fixedtemperature and pressure, then the samples must contain the same number of

freely moving particles (i.e. atoms or molecules).

DEFINITION: Avogadro’s Law

Equal volumes of gases,at the same temperatureand pressure, contain the same num-

ber of molecules.

Tip

1. The value ofR is
   the same for all
   gases


2. All quantities
   in the equation
   pV=nRT must
   be in the same
   units as the value
   ofR. In other
   words, SI units
   must be used
   throughout the
   equation.

You will remember froman earlier section, that we combined different gas law equations to get one

that included temperature, volume and pressure. In this equation, pV = kT, the value of k is different

for different masses of gas. If we were to measurethe amount of gas in moles, then k = nR, where n is

the number of moles ofgas and R is the universal gas constant. The value of R is 8. 3143 J·K−^1 ·mol−^1 ,

or for most calculations, 8. 3 J· K−^1 · mol−^1. So, if we replace k in the general gas equation, we get the

following ideal gas equation.

The joule can be defined as: 1 J = 1 Pa· m^3.

pV = nRT

The following table mayhelp you when you convert to SI units.

Variable Pressure (p) Volume (V) moles (n) universal gas

constant (R)

temperature

(K)

SI unit Pascals (Pa) m^3 mol J.K−^1 .mol−^1 Kelvin (K)

Other units

and conver-

sions

760 mm Hg =

1 atm =

101325 Pa =

101. 325 kPa

1 m^3 =

1 000 000 cm^3 =

1000 dm^3 =

1 000 �

K =◦C + 273

Table 5.1: Conversion table showing different units of measurement for volume, pressure and temper-

ature.

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