Ideal gas Law: The four laws of Boyle, Charles, Gay–Lussac and Avogadro are combined into the Ideal gas law :

P V = n R T

where P is the pressure, V is the volume, n is the number of moles of gas, and T is the temperature in Kelvin. The constant 'R' is called as the universal gas constant which is needed to make all of the relationships fit together. The numerical value of R depends upon the units used. R = 0.0821 L atm mol⁻¹ K⁻¹, R = 8.314 J/mol⁻¹ K⁻¹, R = 8.314 volt coulomb mol⁻¹ K⁻¹.

There are two ways to use the ideal gas law. First, a problem may give three of the four variables and ask that the fourth be calculated. This involves direct substitution of data into the ideal gas law equation. The second way to use the law involves taking a gas under certain initial conditions of P, V, T, and n and changing to some different, final conditions of these four variables. To solve this type of problem the ratio of the ideal gas law equations for the initial and final conditions is written as:

P_iV_i / P_fV_f = n_iRT_i / n_fRT_f

Standard temperature and pressure (STP):

The ideal gas law has four variables P, V, n and T along with the constant R. By defining the standard pressure as exactly 1 atmosphere and the standard temperature as exactly zero degrees Celsius, two of the variables can be stated quickly and easily. Therefore, any gas at STP is understood to have P = 1.00 atm and T = 273K, and only n and V need to be stated in a problem.

Derivation Of the ideal gas equation:

By Charles's law, P ∝ T, at constant volume.
By Gay Lussac's law, V ∝ T, at constant pressure.
By Gay Lussac's law, V ∝ T, at constant pressure.
This gives PV = RT, where R is a constant called universal gas constant. If the volume V contains n moles, the general gas equation become PV = nRT.
This equation is also called equation of state of an ideal gas.

MORE INFO