The empirical formula of a compound was determined to be P2O3. The molecular weight of the compound is 220 g/mol. What is the molecular formula of the compound?
The Correct Answer and Explanation is:
To determine the molecular formula of a compound from its empirical formula and molecular weight, follow these steps:
Step 1: Calculate the molar mass of the empirical formula P₂O₃.
- Atomic mass of phosphorus (P) = 30.97 g/mol
- Atomic mass of oxygen (O) = 16.00 g/mol
- Molar mass of P₂O₃ = (2 × 30.97) + (3 × 16.00)
- Molar mass = 61.94 + 48.00 = 109.94 g/mol
Step 2: Divide the molecular weight by the empirical formula weight.
- Given molecular weight = 220 g/mol
- Empirical formula weight = 109.94 g/mol
- Ratio = 220 / 109.94 ≈ 2
Step 3: Multiply the subscripts in the empirical formula by this ratio.
- Empirical formula = P₂O₃
- Multiply each subscript by 2:
(2 × 2) = 4 for P
(3 × 2) = 6 for O - Molecular formula = P₄O₆
Final Answer: P₄O₆
Explanation:
The empirical formula represents the simplest whole number ratio of atoms in a compound. In this case, the empirical formula P₂O₃ tells us that for every 2 phosphorus atoms, there are 3 oxygen atoms. However, this does not always represent the actual number of atoms in a molecule.
To find the molecular formula, which shows the actual number of each type of atom in a molecule, we need to know the molecular weight of the compound. Once we know both the empirical formula mass and the molecular weight, we divide the molecular weight by the empirical formula mass. This gives us a multiplier. We apply this multiplier to each subscript in the empirical formula to get the molecular formula.
Since the molecular weight is about twice the mass of the empirical formula, it means the molecular formula must have twice as many atoms of each element. That results in the formula P₄O₆.
