From a physical point of view, the "partial pressure" of a component "i" in a gas mixture is defined as the contribution of molecules "i" to the force per unit area that the molecules of the mixture exert on the wall. The relation pi = x[subscript i]P (where x[subscript i] is the mole fraction of "i" and "P" the total pressure) is in agreement with this definition for perfect gases, but not for real gases. For lack of a general macroscopic equation for the partial pressure in real gas mixtures, the product x[subscript i]P is used as "operational partial pressure." Using the virial expansion applied to the equation of state of nonideal gas mixtures truncated to the second order, we obtain an approximate equation of the partial pressures for such mixtures. This equation allows for a comparison with x[subscript i]P as well as for the creation of diagrams expressing the total and partial pressures of the various constituents as a function of composition. Such diagrams prove useful to tackle this delicate subject with students, from a conceptual and from an educational point of view.