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**TriKri** How can I calculate a gas' or a liquid's pressure from it's density and it's temperature? For example, for air, I suppose the pressure $\displaystyle p\$ and the density $\displaystyle \rho$ is proportional, $\displaystyle p\ \propto\ \rho$. Is this true? That would mean that since the unit of pressure is $\displaystyle Pa$ and the unit for density is $\displaystyle kg/m^3$, the unit for pressure per density would be $\displaystyle \frac{Pa}{kg/m^3}\ =\ \frac{Pa\cdot m^3}{kg}$, or $\displaystyle \frac{Nm^3/m^2}{kg}\ =\ \frac{Nm}{kg}$, or $\displaystyle \frac{m^2}{s^2}$. Im just thinking a little bit. And since the atmospherical pressure is $\displaystyle 101\ 325\ Pa$, and the air density in one atm pressure is $\displaystyle 1.2$ to heaped $\displaystyle 1.3\ kg/m^3$ in normal temperatures (here in Sweden). So then we can say that $\displaystyle \frac{p}{\rho}\ =\ \frac{101\ 325 Pa}{1.25\ kg/m^3}\ =\ 81\ 060\ Pa\cdot m^3/kg$

Now I don't know if this is true. And this is only for the temperature $\displaystyle 11^\circ\ C\ \sim\ 52^\circ\ F$.

Then I guess It is not the same thing at all with liquids, not water at least. Since the density for water (in $\displaystyle 4^\circ\ C$) is always $\displaystyle 1\ kg/dm^3$ but the pressure can change. Okay I guess that is not completely true, but almost.