Multiply the volume (in Liters) of the liquid oxygen by 1,000 to convert it to milliliters (ml). In our example 70 L will be converted to 70,000 ml.

Multiply the volume of the liquid oxygen by its density, 1.14 g/ml, to calculate the mass of the compound. In our example, the mass of oxygen is 70,000 ml x 1.14 g/ml or 79,800 g.

Divide the mass of oxygen by its molecular mass to calculate the number of moles. In our example, the oxygen amount is 79,800 g / 32 g/mole = 2,493.75 moles.

Convert temperature in Celsius to Kelvin (K) by adding the value "273.15." In this example, temperature is 20 + 273.15 = 293.15 K.

Multiply the pressure in atm by the factor "101,325" to convert the pressure to the SI unit Pascal (Pa). In our example, Pressure = 101,325 x 1 atm = 101,325 Pa.

Round the molar gas constant R to the fourth digit to obtain 8.3145 J/mole x K. Note that the constant is given in the International System of Units (SI). "J" means Joule, a unit of energy.

Calculate the volume (in cubic meters) of gaseous oxygen using the ideal gas law: multiply the amount of oxygen (in moles) by temperature and the molar gas constant followed by dividing the product by pressure. In our example, Volume = 2493.75 (mole) x 8.3145 (J/mole x K) x 293.15(K) / 101,325 (Pa) = 59.99 cubic meters or 59,990 L.