To calculate the formula weight of magnesium hydroxide, we need to add the atomic weights of each atom in the compound. Magnesium hydroxide has the chemical formula Mg(OH)2.

- The atomic weight of Magnesium (Mg) is approximately 24.305 atomic mass units (amu).

- The atomic weight of Oxygen (O) is approximately 15.999 amu.

- The atomic weight of Hydrogen (H) is approximately 1.008 amu.

Calculating the Formula Weight:

1. Magnesium (Mg): 1 atom x 24.305 amu = 24.305 amu

2. Oxygen (O): 2 atoms x 15.999 amu = 31.998 amu

3. Hydrogen (H): 2 atoms x 1.008 amu = 2.016 amu

Now, we add up the atomic weights:

Formula Weight = 24.305 amu (Mg) + 31.998 amu (O) + 2.016 amu (H)

= 58.319 amu

Therefore, the formula weight of magnesium hydroxide is approximately 58.319 atomic mass units.