Question

relation between pressure and density

Answer 1

For an ideal gas, P = rho R T, where P is the pressure, rho (Greek letter) is density, R is the gas constant for the particular gas, and T is the absolute temperature.  If you hold the temperature constant, then pressure and density both increase in proportion to one another. 

If it's not an ideal gas, then the relation would be more complex, but density would still increase approximately in proportion to pressure (keeping temperature fixed).

Answer 2

P = (rho) (R/MW) T

where P = pressure, rho = mass density, R = universal gas constant, MW = average molecular weight, and T = absolute temperature.