We consider an approximating control design for optimal mixing of a non-dissipative scalar field. in an unsteady Stokes flow. The objective of our approach is to achieve optimal mixing at a given final time T > 0, via the active control of the flow velocity v through boundary inputs. Due to zero diffusivity of the scalar field theta, establishing the well-posedness of its Gateaux derivative requires sup(t is an element of[0,T]) parallel to del theta parallel to (L2) < infinity, which in turn demands the flow velocity field to satisfy the condition integral(0) (T) parallel to del nu parallel to L-infinity(Omega) dt < infinity. This condition results in the need to penalize the time derivative of the boundary control in the cost functional. Consequently, the optimality system becomes difficult to solve (Hu in Appl Math Optim 78(1):201-217, 2018). Our current approximating approach provides a more transparent optimality system, with the set of admissible controls square integrable in space-time. This is achieved by first introducing a small diffusivity to the scalar equation and then establishing a rigorous analysis of convergence of the approximating control problem to the original one as the diffusivity approaches to zero. Uniqueness of the optimal solution is obtained for the two dimensional case.