Behaviour Of Gases
Gas laws are study of any two of quantities like pressure, volume and temperature, when the third is kept constant
B. Charle's Law
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,
- Graph between P and V at temperature T1 and T2 such that T1<T2 are shown below,
- 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
- 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
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.
P = ρRT/M
ρ - mass density of gas.
Dalton law of partial pressures
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- 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.
- Revision Notes