In order to find the pressure, we will use the ideal gas equation which is:
PV = nRT
n is the number of moles. By definition,
moles = mass (m) / molecular weight (Mr)
So, we may substitute the previous equation for "n" in the ideal gas law. Rearranging the equation and substituting (remember to convert kg into grams and the temperature into Kelvin),
P = mRT/VMr
P = (4.04 x 10³ * 0.082 * (273 + 23)) / (50 * 32)
P = 61.3 atm
The pressure of the oxygen gas is 61.3 atm