Mass (m), volume (V), pressure (p), and temperature (T) is of a gas are the measureable properties. The laws which inter-relate these properties are called **gas laws**.

## Perfect Gas or Ideal Gas

The gas whose molecules are point masses (mass without volume) and do not attract each other, is called **ideal** or **perfect gas**. It is a hypothetical concept which can not exist in reality. The gases such as hydrogen, oxygen or helium which can not be liquefied, are called **parmanent gases**.

*Properties of ideal Gas are as follows*

- It strictly obeys Boyle’s law, Charle law and the law of pressure under all consitions of temperature and pressure.
- Its pressure coefficient and the volume coefficient are exactly equal to each other.
- A perfect gas can not be converted into liquid or solid state, because a force of attraction is necessary between the molecules in case of liquid or solid state.

## Ideal Gas law or Equation

The tree laws (Boyle’s law, Charles law and Avogardo’s law) can be combined together in a single equationn which is known as ideal gas equation.

### Gas law

**Boyle’s Law**(pressure-voulme relationship) According to this law, at constant temperature, pressure of a fixed amount of gas varies inversely with its volume, i.e.,

p∝1/V(at constant T) or pV = k (constant) or p1V1=p2V2

at constant temperature, pressure of the gas is directly proportional to the density of a fixed mass of the gas.

i.e., p ∝ d

2. **Charles’ Law **(Temperature-Volume relationship) According to this law, at constant pressure, the volume of a fixed mass of a gas is directly proportional to its absolute temperature i.e., decreases with a decrease in temperature.

V∝T (at constant p) or V1/T1 = V2/T2

The lowest hypotheticcal or imaginary temperature at which gases are supposed to occupy zero volume, is called **absolute zero**.

3. **Gay Lussac’s Law** (Pressure-Temperature relationship) According to this law, at constant volume, pressure of a fixed amount of a gas varies directly with the temperature, i.e.,

p ∝ T or p/T= constant or p1/T1=p2/T2

4. **Avogadro’s Law **(Volume-Amount relationship) According to this law, equal volumes of all the gases under the same conditions of temperature and pressure contain the equal number of molecules, i.e.,

V ∝ n (at constant T and p)

where, n=number of molecules

at STP, gram molecular mass or 1 mole of gas occupies volue of 22.4 L.

Number of molecules in one mole of a gas has been determined to be 6.022 x 10^{23} This number is known as **Avogadro’s constant**

5. **Combined Gas Law **This is the relationship for the simultaneous variation of the variables. If temperature, volume and pressure of a fixed amount of gas vary from T1, V1 and p1 to T2, V2 and p2 then we can write

pV/T=nR or p1V1/T1=p1V2/T2

6. **Dalton’s Law of Partial Pressure** It states that the total pressure exerted by gaseous mixture of two or more non-reacting gases is equal to the sum of the partial pressure of each individual component in a gas mixture, i.e.,

*p*_{total}= p_{1}+p_{2}+p_{3}…..p_{n} (at constant T, V)

where, p_{1}, p_{2}, p_{3} … are the partial pressures of individual gases.

7. **Graham’s Law of Diffusion** According to this law, at constant temperature and pressure, the rate of diffusion (r) of a gas is inversely proportional to the square root of its density (d) i.e.,

(**Diffusion **is the process of spontaneous mixing of different gases and the volume of a gas diffused per unit time, is called **rate of diffusion**)

*This law is applicable *

- In the production of marsh gas
- In the separation of gaseous mixture
- In the determination of vapour densities of gases
- In the Separation of Isotopes

### Indeal Gas Equation

At constant T and n;

V ∝ 1/*p* (Boyle’s Law)

At constant *p* and n;

V ∝ T (Charles’ Law)

At constant *p* and T;

V ∝ n (Avogadro’s law)

V ∝ nT/*p *or V = Rx*n*T/*p*

where, R is proportionality constant, On rearranging the above equation, we obtain

*p*V=*n*RT (ideal gas equation)

R=*p*V/*n*T

R is called universal gas constant and has vol. 8.314 J mol^{-1} K^{-1} or 0.0821 L atm mol^{-1} K^{-1}

Ideal gas equation is a relation between four variables and it describes the state of any gas, therefore, it is also called **equation of state**.