A 5.71 g of an unknown metal sulfate with formula M2SO4, was dissolved in water and treated with excess aqueous barium nitrate, resulting in the precipitation of all the sulfate ions as barium sulfate. The precipitate was collected, dried, and found to weigh 3.68 g. Determine the atomic mass of M, and identify M

What you need to do my friend is write the chemical equation of dissociation of your acid AH with water, with initial concentration co, then consider that a fraction α reacts with water. Concentration and Ka are big enough so that you can consider that
1−α≈1
 Then write expression of Ka, then solve for α. You should find about 18.4% I hope this works really good for you