According to Le Châtelier’s Principle, when the condition of the system changes the equilibrium will shift to compensate for the changes. When temperature decrease, the pressure will also decrease and this will shift the equilibrium to the side with exothermic reaction or less molecule count.
The question is not giving the equilibrium reaction of oxygen, so I assume its
O2 + Hb = HbO2
Since the number of molecules in the right side is lower, then the number of oxygen will decrease since the reaction will shift to the right.
One thing that does not change is the chemical composition of water, which is still H2O. And maybe mass, if all of the particles remain inside the beaker, which was never mentioned in the question so I am not sure.
