Question

density of Oxygen gas at 25 degrees celsius is 4.458 MG per litre at one atmosphere at what pressure will oxygen have the density twice the value

Answer 1

p = R ρ T
p is pressure,
ρ is density
T is temperature (Kelvin),
R is the specific gas constant, which is dependent on individual gases.

Density of oxygen gas at 298 K is 1.458mg/L at 1 atm
1 atm = R×1.458 mg/L×298 K
Since the temperature and the gas constant cannot be changed, the pressure will have to be doubled to make density twice the origina value at same temperature.