Two different proteins X X and Y Y are dissolved in aqueous solution at 37 ∘C ∘ C . The proteins bind in a 1:1 ratio to form XY X Y . A solution that is initially 1.00 mM m M in each protein is allowed to reach equilibrium. At equilibrium, 0.19 mM m M of free X X and 0.19 mM m M of free Y Y remain.