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The density of mixture can be calculated from its composition as composition:

\rho = \frac{P}{R \, T} \cdot \sum_\alpha \ x_\alpha \cdot M_\alpha

where

	mole fraction of chemical component
	molar mass of chemical component
	mixture temperature
	mixture pressure
	gas constant

The relative density then can be found as:

\gamma =   \left( \sum_\alpha \ x_\alpha \cdot M_\alpha \right) \big / M_{\rm ref}

where is the molar mass of the reference fluid (usually, water or air depending on the context).

The does not depend on mixture temperature and pressure explicitly but it does implicitly through the corresponding value of the reference fluid molar mass .

The usual practice is to calculate the fluid mixture density at STP and then use pressure-density correlations to predict its properties at various temperature and pressure.

Here is the list of the molar mass for popular pure petroleum hydrocarbons : Alkanes.

See Also