Behaviour Of Gases

Gas Laws

    Gas laws are study of any two of quantities like pressure, volume and temperature, when the third is kept constant

    A. Boyle's law

  • "At constant temperature, the volume of a given mass of gas is inversely proportional to pressure." Thus
    V ∝ 1/P
    Or, PV = constant(1)
  • If P1, V1are initial pressure and volumes and P2, V2be final values then,
    P1V1= P2V2
  • Graph between P and V at temperature T1 and T2 such that T1<T2 are shown below,

    Boyle's law
  • Graph above shows that Boyle's law is strictly not obeyed by gases at all values of P and T but it obeys this law only at low pressure and high temperature i.e., at law density
B. Charle's Law
  • Charle's Law is stated as follows :
    "When pressure of a gas is constant the volume of a given mass of gas is directly proportional to its absolute temperature".
    V/T = Constant(2)
  • Graph between V and T is

    Charle's Law
    This graph shows that experimental graph deviates from straight line. Theoretical and experimental graphs are in agreement at high temperature.

Ideal gas equation

  • We can combine Boyle' law eqn (1) and Charle's' law eqn (2) in to a single eqn i.e.,
    PV/T = Constant(3)
    If n moles is the mass of gas then we write
    PV = nRT(4)
    where, n is number of moles of gas, R=NAKB is the universal constant known as gas constant and T is the absolute temperature.
  • A gas satisfying eqn (4) at all values of pressures and temperatures is said to be an ideal gas
    now no of moles of gas
    n = m/M = N/NA
    m - mass of gas containing N molecules
    M - molar mass
    NA - Avogadro' number.
    From this,
    P = ρRT/M
    ρ - mass density of gas.

Concept Map of Ideal Gas

Dalton law of partial pressures

  • Consider a mixture of non-interacting ideal gases with n1 moles of gas 1,n2 of gas 2 and so on
  • Gases are enclosed in an enclosure with volume V, temperature T and pressure P.
  • Equation of state of mixture
    PV = (n1+ n2)RT
    orP = n1RT/V + n2RT/V + - - - -
    = P1 + P2 + - - --
    P1 = n1RT/V
    is pressure the gas 1 would exert at same V and T if no other gases were present in the enclosure. This is know as law of partial pressure of the gases.
  • The total pressure of mixture of ideal gases is sum of partial pressures of individual gases of which mixture is made of.

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