145.8g
Explanation:
Given parameters:
Number of moles of magnesium hydroxide  = 2.5mol
Unknown:
Mass of Mg(OH)â‚‚ Â = ?
Solution:
To solve this problem we use the expression below;
 Mass of Mg(OH)₂ = number of moles x molar mass
 Molar mass of Mg(OH)₂ = 24.3 + 2(16 + 1)  = 58.3g/mol
 Mass of Mg(OH)₂ = 2.5 x 58.3  = 145.8g