Equilibrium constant - K value

Dear friends...

Could you shed some light on this?

If we have

[math]K = \frac{y}{x}[/math]

would it be correct to say that such an equation could be applied to calculations involving either ideal gas or real gas?

Can we have (????)

[math] K_{i} = \frac{y_{i}}{x_{i}} = \frac{\phi^{vap}_{i}}{\phi^{liq}_{i}} [/math]

[math]\phi[/math] = fugacity coefficient

are they always interchangeable?

Thank you.

Marcio