The formula for the compound formed by barium ion (Ba²⁺) and hydroxide ion (OH⁻) is Ba(OH)₂.

Here's how to determine it:

* Balance the charges: Barium has a +2 charge (Ba²⁺) and hydroxide has a -1 charge (OH⁻). To balance the charges, you need two hydroxide ions for every barium ion.

* Write the formula: This gives you the formula Ba(OH)₂.

This compound is called barium hydroxide.