A 1.0 L flask contains 0.40 g O2 at a temperature of 20�C. What is the pressure (in atm) inside the flask?

For this you need the ideal gas equation PV=nRT (pressure x volume=moles of gas x constant x temperature in Kelvin [Celsius + 273]) You need to convert grams into moles for n and enter the other numbers to solve for P which is ATMs of pressure. The constant R is 0.0821 x 10^14.