Dalton's law is formulated as follows: the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of these gases. The partial pressure is the pressure of a given gas as if it alone occupied.

Explanation

In thermodynamical equilibrium the temperature of all the components of a mixture of gases is the same. The pressure of the individual component is
`p_1 = n_1 kT = N_t kT//V, p_2 = n_2 kY = N_2kT//V`, etc.
The sum of partial pressures is
`p_1 + p_2 + p+  = (N_1 + N_2 + N_3 + …) (kT)/V`
Since the total number of molecules N is the sum of tho numbers of molecules of all the components making up the mixture, `N = N_1 + N_2 + N_3 + ….` the sum of the partial pressures is equal to the pressure of the gas. :
`p_1 + p_2 + p_3 +  . = NkT//V = p`.