The percentage by mass of calcium in Ca(OCI)2 is [Ca = 40.0; CI = 35.5; O = 16.0]

To calculate the percentage by mass of calcium in Ca(OCl)2, we need to first find the molar mass of the compound, which is the sum of the atomic masses of its elements. Molar mass of Ca(OCl)2 = (1 × atomic mass of Ca) + (2 × atomic mass of O) + (2 × atomic mass of Cl) = (1 × 40.0) + (2 × 16.0) + (2 × 35.5) = 142.0 g/mol Next, we need to find the mass of calcium in one mole of Ca(OCl)2. Mass of calcium = (1 × atomic mass of Ca) = 40.0 g/mol Finally, we can calculate the percentage by mass of calcium in Ca(OCl)2 by dividing the mass of calcium by the molar mass of the compound and multiplying by 100. Percentage by mass of calcium in Ca(OCl)2 = (mass of calcium / molar mass of Ca(OCl)2) × 100 = (40.0 / 142.0) × 100 = 28.2% (rounded to one decimal place) Therefore, the correct answer is (A) 28.0%.