The empirical formula of a compound represents the simplest whole number ratio of elements present in the compound. To determine the empirical formula of the compound containing only phosphorus and oxygen, we need to first calculate the mass of oxygen in the compound.

Given that the compound is 56.35% phosphorus, the mass of oxygen can be calculated as follows:

Mass of oxygen = Total mass of compound - Mass of phosphorus

Mass of oxygen = 100% - 56.35% = 43.65%

Now, we need to convert the percentage composition of each element into the number of moles. Molar mass of phosphorus (P) is 30.97 g/mol, and molar mass of oxygen (O) is 16.00 g/mol.

Moles of phosphorus = (56.35 g P) / (30.97 g/mol) = 1.82 mol P

Moles of oxygen = (43.65 g O) / (16.00 g/mol) = 2.73 mol O

Next, we need to find the simplest whole number ratio of moles by dividing both the number of moles by the smallest number of moles:

Moles of phosphorus (simplified) = 1.82 mol P / 1.82 mol = 1

Moles of oxygen (simplified) = 2.73 mol O / 1.82 mol ≈ 1.5

To obtain whole numbers, we can multiply both the simplified moles by 2:

Moles of phosphorus (final) = 1 × 2 = 2

Moles of oxygen (final) = 1.5 × 2 = 3

Therefore, the empirical formula of the compound is P₂O₃.