Final answer:
The formula mass of the compound formed between Al³⁺ and Cl⁻ is 133.5 u, which corresponds to the molar mass of AlCl₃ (Aluminum Chloride) when rounded to the nearest tenth.
Step-by-step explanation:
The formula mass of the compound formed between Al³⁺ and Cl⁻ can be calculated by first determining the correct formula of the compound and then adding up the atomic masses of the constituent elements. Aluminum chloride is AlCl₃. The molar mass of aluminum (Al) is approximately 26.98 u and the average atomic mass of chlorine (Cl) is approximately 35.45 u (taking into account the natural isotopic abundance of Cl-35 and Cl-37). Remembering that there are three chlorine atoms for every aluminum atom in the compound, we calculate the molar mass as follows:
Molar mass = 1(26.98 u) + 3(35.45 u) = 26.98 u + 106.35 u = 133.33 u
Thus, the correct answer is option d, which states the formula mass is 133.5 u, rounding to the nearest tenth.