TITRATION QUESTIONS -- Short Answer -- PLEASE HELP !?

Question

1) Arsenic acid H3 As O4 is a tribasic acid. 25.0ml of a solution of the acid requires 35.7ml of a solution of sodium hydroxide of concentration 0.100 mol/L for neutralization. What is the concentration of the acid? and give the the equation for it.


2) A piece of iron wire weighs 0.2756g. It is dissolved in acid, reduced to the Fe2+ oxidation state, and titrated with 0.0200 mol/L potassium dichromate solution. What is the percentage purity of the iron wire? Use half-equations


3) 225g of iron wire was treated with sulphuric acid to convert it to Fe2+. All of the Fe2+ was placed in a 250ml flask and the solution made up to the mark with water. 25ml of this solution was titrated with a 0.00185 mol/L solution of potassium dichromate(VI). The volume required was 31.0ml. Calculate the percentage of iron in the wire. Use half equations.

Answer 1

1) Normality of H3AsO4 = 35.7x0.1/25 = 0.1428 N

So, molarity = 0.1428/3 = 0.0476 M

2) Cr2O7(2-) + 6e- + 14H+ -> 2Cr(3+) + 7H2O

Fe2+ -> Fe3+ + e-

Vol. of the soln. used ???