Final answer:
The molecular formula for the unknown compound is Ba3Cl2.
Step-by-step explanation:
To determine the molecular formula of the unknown compound, we need to find the empirical formula and the molar mass of the compound. The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of atoms in one molecule of the compound.
Based on the given percentages, we can assume 100g of the compound. This means we have 65.9g of barium (Ba) and 34.1g of chlorine (Cl). To find the empirical formula, we calculate the moles of each element: 65.9g Ba / 137.33 g/mol = 0.479 mol Ba and 34.1g Cl / 35.45 g/mol = 0.962 mol Cl. The ratio of Ba to Cl is approximately 1:2, so the empirical formula is BaCl2.
To determine the molecular formula, we need to compare the empirical formula mass to the experimental formula mass. The empirical formula mass of BaCl2 is approximately 137.33 g/mol + (2 x 35.45 g/mol) = 208.23 g/mol. The experimental formula mass is given as 621.31 g/mol. Dividing the experimental formula mass by the empirical formula mass gives us a factor of 2.985. This means the molecular formula is approximately 2 times the empirical formula, resulting in Ba2Cl4. However, this is not an available option, so the closest matching option is D) Ba3Cl2.