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Ideal Gas Law

What is Ideal Gas Law?

Ideal gas law or perfect gas law represents the mixed relationship between pressure, volume, the temperature of gases for learning the physical properties of the gas molecule in physics or chemistry. The ideal gas equation balancing these state variables in terms of universal gas constant (R). The ideal or perfect gas law formula can use for calculating the value of pressure, volume, temperature, diffusion or effusion, concentration, and the number of gas molecules per unit volume or density. Boyle’s in 1662, Charles’s in 1787, and Avogadro law give the general derivation formula of the ideal or perfect gas equation, and the kinetic theory of gas provides the properties of ideal gases.

The four thermodynamics variables in the gas laws for the ideal or perfect gases are pressure, volume, temperature, and mole number. Some of these depend on the mass of the system while others are independent of the mass.

  • In thermodynamics, the property which proportional to the mass of the system is called intensive property.
  • Property of the system which is independent of the mass of the system is called intensive property.

In ideal gas law, the volume is an intensive property but temperature, pressure are extensive properties in thermodynamics derivation.

Ideal Gas Law Formula Derivation

Ideal gas law formula or equation derivation in chemistry or physics

Boyles law V ∝ 1/T, when n and T are constant. Charles law, V ∝ T, when n and P are constant. Avogadro’s law, V ∝ n, when P and T are constant. When all the variables of gas laws are taken into account, we find out the mathematical expression of the ideal gas law equation, PV = nRT, where R = universal gas constant.

Ideal gas law defines the relation between pressure, volume, temperature, and composition of gases. But the equation is found to hold most satisfaction when pressure is low or tense to zero. At ordinary temperature and pressure, the equation is found to deviated about 5%. Therefore, the real or Van der Waals gas obeys ideal gas law only at low pressures and very high temperatures.

Universal Gas Constant Value

Universal constant values unit and dimension can be calculated from ideal gas law, PV = nRT. At NTP 1 mole gases at 1-atmosphere pressure occupied 22.4 lit of volume. From the ideal gas equation, R = PV/nT = (1 atm × 22.4 lit)/(1 mol × 273 K) = 0.082 lit atm mol-1 K-1.

Value of universal gas constant in CGS-unit

In CGS units, pressure = 1 atm = 76 × 13.6 × 981 dyne cm-2 and volume = 22.4 liter = 22.4 × 103 cm3. Putting the values of P, V, T, and n in ideal gas law, we have universal gas constant (R) = (7.6 × 13.6 × 981 × 22.4 × 103)/(1 × 273) = 8.314 × 107 dyne cm2 mol-1 K-1 = 8.314 × 107 erg mol-1 K-1, where dyne cm2 = erg.

Value of universal gas constant in SI-unit

The Values of universal constant (R) in CGS-system = 8.314 × 107 erg mol-1 K-1. But 1 J = 107 erg. The universal constant in SI units = 8.314 J mol-1 K-1. Agin from specific heat relation, 4.18 J = 1 calorie. Therefore, universal gas constant from ideal gas law equation = (8.314/ 4.18) cal mol-1 K-1 = 1.987 calories mol-1 K-1 ≃ 2 calories mol-1 K-1.

Importance of Ideal Gas Law

For n mole ideal gases, PV = nRT or R = PV/nT. The unit of universal gas constant = (unit of pressure × unit of volume)/(amount of gas molecule × unit of temperature). Here, the unit of pressure = force length-2 and volume = length3. The unit of R = (force × length)/(amount of gas molecule × unit of temperature), where force × length = work or energy.

From the general definition of the ideal gas law equation in learning chemistry or physics, R = energy per mole per kelvin or the amount of work or energy that can be obtained from one mole of gases when its temperature is raised by one kelvin.

Ideal Gas Density Calculation

The ideal gas law for n mole, PV = nRT = (g/M) × RT, where g = weight in gram, M = molar mass. Therefore, P = dRT/M, where d = density = g/V. The ideal gas law formula used to find out the density of gases in science from the known molar mass of the mixed gases.

Problem: The density of ammonia at 5 atmosphere pressure and 30°C temperature 3.42 g lit-1. How can we calculate the molar mass of ammonia from the ideal gas equation?

Solution: Molar mass (M) = dRT/P. Molecular mass of ammonia, MNH3 =(3.42 × 0.082 × 303)/5 = 16.99 g mol-1≃ 17 g mol-1.

Number of Molecules Per Unit Volume

PV = nRT = (N/N0) × RT
where N = number gas molecules present
N0 = Avogadro number = 6.023 × 1023

∴ P = (N/V) × (R/N0) × T = N′KT
where N′ = number molecules per unit volume.
k = Boltzmann constant = R/N₀
= 1.38 × 10-16 erg molecules-1 K-1

Problem: Estimate from the ideal gas equation, the number of gaseous molecules left in a volume of 1 mi-liter if it pumped out to give a vacuum of 7.6 × 10⁻³ mm of Hg at 0°C.

Solution: Volume (V) = 1 ml = 10-6 dm3, Pressure (P) = 7.6 × 10-3 mm Hg = 1.01235 × 10-3 kPa. The number of gas molecules, N’ = (1.01235 × 10-3)/(1.38 × 10-9 × 273) = 2.68 × 10-11.

Total Pressure of Gas Mixture

Assuming perfect behavior or obeying ideal gas law to finds out the mixed pressure exerted by 2 g organic compound like methane and 3 g carbon dioxide gas molecules in a vessel of 5-liter capacity at 50°C.

nCH4 = 2/16 = 0.125
nCO2 = 3/44 = 0.0682
Total moles (n1 + n2) = (0.125 + 0.0682)
= 0.1932
∴ Ptotal = (0.1932 × 0.082 × 323)/5 atm
= 5.30 atm

The total pressure of mixed ideal gas molecules can be derived from ideal gas law and uses for the calculation of the mixed pressure for different gases like oxygen, nitrogen, carbon dioxide, hydrocarbon, etc.