What concentration of sulfuric acid will make a solution of pH=3.4

Answer 1

From your earlier question about H2SO4, let's take the simple route.
pH = -log(H^+) so for pH of 3.4 then (H^+) = 3.98E-4
H2SO4 ==> 2H^+ + SO4^2-
So half that is what you want.