Reaction
2Fe(OH)₃ + 3H₂SO₄⇒Fe₂(SO₄)₃ + 6H₂O
because the limiting reactant is Fe(OH)₃, then the moles of all compounds will be based on moles of Fe(OH)₃
mole H₂SO₄ = 3/2 x mole Fe(OH)₃ = 3/2 x 3 mole = 4.5 mole
remaining mole of H₂SO₄ (as an excess reactant) = 6.4 - 4.5 = 1.9 mole
