Find the formula unit mass of magnesium hydroxide, Mg(OH)₂. Atomic mass — Mg = 24 u; O = 16 u; H = 1 u.
58 u
Explanation
As explained in the textbook, ionic compounds do not form molecules but 3-D crystals, so we calculate their formula unit mass instead of molecular mass. For compounds with polyatomic ions like Mg(OH)₂, the subscript outside the bracket applies to the entire hydroxide (OH⁻) ion. Following the method shown in Example 9.7 for Ca(NO₃)₂, we multiply the atomic masses of the elements inside the bracket by their respective subscripts, multiply the result by the subscript outside the bracket, and then add the mass of the remaining cation.
Solution Steps
Step 1: Identify the formula and given atomic masses: Mg(OH)₂, where Mg = 24 u, O = 16 u, H = 1 u.
Step 2: Set up the calculation for formula unit mass, treating the (OH)₂ group as shown in the textbook examples: Formula unit mass = (24 u × 1) + {(16 u × 1) + (1 u × 1)} × 2
Step 3: Simplify the terms inside the brackets: = 24 u + (17 u) × 2
Step 4: Multiply and add to get the final result: = 24 u + 34 u = 58 u