The oh - concentration in a 1.0  10-3 m ba (oh) 2 solution is

The Ionization of Ba (OH) ₂ is as follow,

Ba (OH) ₂ → Ba²⁺ + (OH⁻) ₂

Or,

Ba (OH) ₂ → Ba²⁺ + OH⁻ + OH⁻

As,

Concentration of Ba (OH) ₂ is 1.0 * 10⁻³, and each Barium Hydroxde contains two OH⁻ then concenteration of OH⁻ is calculated as,

[OH⁻] = 2 * 1.0 * 10⁻³

[OH⁻] = 0.002 M

Or,

[OH⁻] = 1.0 * 10⁻³