Final answer:
The number of grams of Mg(OH)₂ formed from the reaction cannot be calculated without the correct chemical equation involving Mg and H₂O. An error in the provided chemical species (Mg₂⁺Si) prevents the calculation.
Step-by-step explanation:
To calculate the number of grams of Mg(OH)₂ formed when 90.4 g of Mg₂⁺Si react with excess H₂O, we would need the chemical equation for the reaction. However, it seems that Mg₂⁺Si may be an error since Mg₂⁺ doesn't clearly represent a common chemical species. Assuming that the reaction in question is between magnesium metal (Mg) and water (H₂O) to form magnesium hydroxide (Mg(OH)₂), we would first need to correct the chemical formula before calculating the mass of Mg(OH)₂ formed.
The balanced equation would look like:
Mg + 2 H₂O → Mg(OH)₂ + H₂
After determining the molar masses, we would use stoichiometry to convert grams of Mg to moles and then to grams of Mg(OH)₂. However, since the question provided contains uncertainties regarding the initial reactant, providing a final answer is not possible without the correct chemical formula for the reaction in question.