Find the pressure, in atmosphere, exerted by a mixture which contains three gases with partial pressures of 448 torr, 321 mm hg and 1.72 atm

From my understanding, the main formula you would be using is Dalton's Law of Partial Pressure in order to find the final pressure.
The formula for that is PT=P1, P2, P3etc. 

You also want ALL of your pressures in atmospheres, so atm. Your first step is to convert all of you gases into atmospheres. What you need to remember is for every 760 mmHg, you also have 760 torrs. In other words, there equal to one another

448 torr x (1 atm/760 torr)=0.58947 atm
321 mmHg x (1 atm/760 mmHg)=0.42236 atm
And your last partial pressure is already in atmospheres

So all you need to do is add your numbers together in order to figure out your total pressure.

Hope this helps, if not I'm sorry :(

Because a model would make the investigation easier to understand

I’d have to say no because without oxygen in the water the fish would all be dead. so if you want to fish for dead fish, pick the ponds with the lowest oxygen concentrations.