Density of gas mixture

[brajh11]

Anybody knows how can i calculate the density of:

Light heating oil (C12H23): 840 kg/m3 and air: 1.185 kg/m3

The system is: oil burner in a small burning device and i want to calculate what density is in the inside the burning device.

I want to know the density for LAMBDA: 1.2

Operating conditions:
Pressure of oil injection is: 12 bar
Max. pressure of burning device is: 2 bar

[edgepflow]

Consider as total mass:

mass = density X volume = den X V

den (Total) Vol (Total) = den (oil) V (oil) + den (air) V (air)

and solve for den (Total)