Explanation:

When two compounds have the same empirical formula but different molecular formulas, it means that they have the same relative number of atoms but different actual number of atoms in their molecules. This can occur when the compounds have different arrangements or structures.

Empirical formula:
The empirical formula gives the simplest whole number ratio of atoms in a compound. It does not provide information about the actual number of atoms present in the molecule.

Molecular formula:
The molecular formula gives the actual number of atoms of each element in a molecule of the compound.

Example:
Let's consider an example to understand this concept. Take two compounds, Compound A and Compound B, both having the empirical formula CH₂O.

- Compound A: Molecular formula = C₆H₁₂O₆ (Glucose)
- Compound B: Molecular formula = C₁₂H₂₄O₁₂ (Fructose)

In this example, both compounds have the same empirical formula, CH₂O, which shows the ratio of atoms of carbon, hydrogen, and oxygen. However, the actual number of atoms in the molecules is different.

Reasoning:
The molecular weight of a compound is the sum of the atomic weights of all the atoms in its molecular formula. Since the molecular formulas of the two compounds are different, their molecular weights will also be different. Therefore, the correct answer is option 'B' - different molecular weight.

The other options can be ruled out based on the given information:
- Option 'A' (different percentage composition) is not necessarily true because compounds with the same empirical formula can have the same percentage composition.
- Option 'C' (same viscosity) is not related to the empirical and molecular formulas of compounds.
- Option 'D' (same vapor density) is not necessarily true because the vapor density of a compound depends on its molecular weight and molecular formula.