Which of the following substances have an enthalpy of formation equal to zero? Cl 2 (g), H 2 (g) , N 2 (l) , and Cl * (g)

Large helium-filled balloons are used to lift scientific equipment to high altitudes. what is the pressure inside such a balloon if it starts out at sea level with a temperature of 10.0ºc and rises to an altitude where its volume is twenty times the original volume and its temperature is – 50.0ºc ?