A sample containing 0.0500mol of Fe2(SO4)3 is dissolved in enough water to make 1.00L of solution. This solution contains hydrated SO4 2- and Fe(H2O)6 3+ ions. The latter behaves as an acid:

Question

A sample containing 0.0500mol of Fe2(SO4)3 is dissolved in enough water to make 1.00L of solution. This solution contains hydrated SO4 2- and Fe(H2O)6 3+ ions. The latter behaves as an acid: 
Fe(H2O)6 3+ --> Fe(H2O)5OH 2+ + H+

a) Calculate the expected osmotic pressure of this solution at 250†2C if the above dissociation is neglibile. 
b) The actual osmotic pressure of the solution is 6.73 atm at 250†2C. Calculate Ka for the dissociation reaction of Fe(H2O)6 3+.

Thanks in advance!

Formulas to consider:
Osmotic pressure = MRT
R = 0.08206 --> constant
T = 298 K