The molecular formula `P_(2)O_(5)` means that

Options

A. a molecule contains 2 atoms of P and 5 atoms of O
B. the ratio of the mass of P to the mass of O in the molecule is 2 : 5
C. there are twice as many P atoms in the molecule as there are O atoms
D. the ratio of the mass of P to the mass of O in the molecule is 5 : 2.

Explanation

To solve the question regarding the molecular formula \( P_2O_5 \), we will analyze what this formula indicates step by step.

### Step 1: Understand the Molecular Formula
The molecular formula \( P_2O_5 \) indicates the types and numbers of atoms present in a molecule. Here, "P" stands for phosphorus and "O" stands for oxygen.

### Step 2: Count the Atoms
In the formula \( P_2O_5 \):
- The subscript "2" next to phosphorus (P) means there are **2 atoms of phosphorus**.
- The subscript "5" next to oxygen (O) means there are **5 atoms of oxygen**.

### Step 3: Analyze the Options
Now, let's analyze the given options based on our understanding:
- **Option A**: A molecule contains 2 atoms of phosphorus and 5 atoms of oxygen.
- This is **correct** because we have already counted the atoms.

- **Option B**: The ratio of mass of phosphorus to the mass of oxygen in the molecule is 2:5.
- To check this, we need to calculate the mass of phosphorus and oxygen:
- Mass of phosphorus: \( 2 \times 31 \, \text{g/mol} = 62 \, \text{g/mol} \)
- Mass of oxygen: \( 5 \times 16 \, \text{g/mol} = 80 \, \text{g/mol} \)
- The ratio of mass of phosphorus to mass of oxygen is \( 62:80 \) or \( 31:40 \), which is not \( 2:5 \). Therefore, this option is **incorrect**.

- **Option C**: There are twice as many P atoms in the molecule as there are O atoms.
- This statement is false because there are 2 phosphorus atoms and 5 oxygen atoms. The ratio is not 2:5 but rather 2:5, which does not indicate that phosphorus is twice as much as oxygen. Thus, this option is **incorrect**.

- **Option D**: The ratio of mass of P to the mass of O in the molecule is 5:2.
- As calculated earlier, the mass ratio is \( 31:40 \), which does not correspond to \( 5:2 \). Therefore, this option is also **incorrect**.

### Conclusion
Based on the analysis, the correct answer is:
- **Option A**: A molecule contains 2 atoms of phosphorus and 5 atoms of oxygen.