If two compounds have the same empirical formula but different molecular formula, they must have

Options

A. different percentage composition
B. same viscosity
C. different molecular masses
D. same vapour density.

Explanation

To solve the question, we need to analyze the relationship between empirical formulas and molecular formulas. Here’s a step-by-step breakdown:

### Step 1: Understand Empirical and Molecular Formulas
- **Empirical Formula**: This represents the simplest whole-number ratio of the elements in a compound. For example, the empirical formula for glucose (C6H12O6) is CH2O.
- **Molecular Formula**: This shows the actual number of atoms of each element in a molecule. For glucose, the molecular formula is C6H12O6.

**Hint**: Remember that the empirical formula is a simplified version of the molecular formula.

### Step 2: Analyze the Given Information
- The question states that two compounds have the same empirical formula but different molecular formulas. This means that while they share the same ratio of elements, they differ in the total number of atoms.

**Hint**: Think about how two compounds can have the same ratio of elements but different total amounts of those elements.

### Step 3: Identify the Implications of Different Molecular Formulas
- If two compounds have the same empirical formula but different molecular formulas, it implies that their molecular masses must also differ. This is because the molecular mass is calculated based on the total number of atoms in the molecular formula.

**Hint**: Consider how molecular mass is affected by the number of atoms in the molecular formula.

### Step 4: Evaluate the Options
- The options provided are:
1. Different percentage composition
2. Same viscosity
3. Different molecular masses
4. Same vapor density

- Since the compounds have different molecular formulas, they will have different molecular masses.

**Hint**: Look for the option that directly relates to the difference in molecular formulas.

### Step 5: Conclusion
- The correct answer is that if two compounds have the same empirical formula but different molecular formulas, they must have **different molecular masses**.

### Final Answer
- **Different molecular masses** (Option C).