How can you calculate partial pressure?

By means of ##”Dalton’s Law of Partial Pressures“##.
In a gaseous mixture, the pressure exerted by a component, ##P_A##, is the same as the pressure it would exert if it ALONE occupied the container. The total pressure is the sum of the individual partial pressures.
##P_”Total”## ##=## ##P_A + P_B + P_C + P_D……..##
And if we assume ideality,
##=## ##(n_ART)/V+(n_BRT)/V+(n_CRT)/V+(n_DRT)/V+##
Thus, ##P_”Total”## ##=## ##(RT)/V{n_A + n_B + n_C + n_D + }##
And ##P_A## ##=## ##(RT)/Vxxn_A/{n_A + n_B + n_C + n_D + }##
Thus the ##P_Aprop”Mole fraction of Gas A”##, with the constant of proportionality ##(RT)/V##.