How many hydroxide ions are in 2.5 mol mg(oh)2? ?

We can see from the chemical formula of magnesium hydroxide that each mole contains 2 moles of OH ions. So the molar ratio of magnesium hydroxide to hydroxide ions becomes 1 : 2.
If there are 2.5 moles magnesium hydroxide present, the moles of hydroxide ions will be:
2.5 * 2 = 5 moles of hydroxide ions

Each mole of a substance contains 6.02 x 10²³ particles. Thus, the total number of ions will be:
5 x 6.02 x 10²³
= 3.01 x 10²⁴ hydroxide ions

The answer is b on plato.