What is the purpose of having a water-tight covering?

An enzymatic hydrolysis of fructose-1-p, fructose-1-p(aq) + h2o(l) - fructose (aq) + pi (aq) was allowed to proceed to equilibrium at 25°c. the original concentration of fructose-1-p was 0.2 m, but when the system had reached equilibrium, the concentration of fructose-1-p was only 6.52 x 10^-5 m. calculate the equilibrium constant for this reaction and the free energy of hydrolysis of fructose-1-p.