The gas inside a standard party balloon is generally compressed 3% to inflate the balloon. This wipes out even the theoretical buoyancy of nitrogen. And trust me, there was never any practical buoyancy to begin with. You’d need a ridiculously large balloon in a room with impossibly still air and impossibly null thermal gradients to even measure the buoyancy of nitrogen vs air. The buoyancy of nitrogen vs air would never be perceptible to human senses in any real-world setting.
It would be the same as just filling the balloon with air.
I'm 20+ years out of college and I asked a question specifically because I was unsure. Give me a break. I'm sorry if "party balloon buoyancy physics" wasn't the part of my college classes that stuck with me.
You probably do, actually! People constantly underestimate the grand utility of their basic education.
At near-atmospheric pressure and typical ambient temperatures, the ideal gas equation (PV=nRT) from introductory physics works very well and indicates that a 3% overpressure would make gases 3% more dense (linear direct proportionality). At some threshold of high pressures/ low temperatures, you'd want to switch your equation of state (EOS) from ideal gas law to something else. Peng-Robinson would be a good choice for a non-polar gas like Nitrogen, if its >10-50 atm pressure and/or < -50C temperature.
At 20 degC, 1.00atm to 3kPa gauge pressure, ideal gas law predicts nitrogen would increase in density by 2.9608%. Whereas Peng-Robinson predicts it would increase in density by ever-so-slightly more, 2.9623%. This is truly negligible, so better to use the simples EOS for explainability (which would be the ideal gas law).