Molecular formula can be calculated if the molar mass is known after having an empirical formula.

To understand why the correct answer is option 'A' (Molecular), let's first define what empirical and molecular formulas are:

Empirical formula: The empirical formula is the simplest ratio of the atoms present in a compound. It gives the relative number of atoms of each element in a compound. For example, the empirical formula of glucose is CH2O, which means that for every carbon atom, there are two hydrogen atoms and one oxygen atom.

Molecular formula: The molecular formula represents the actual number of atoms of each element in a molecule of the compound. It can be the same as the empirical formula or a multiple of it. For example, the molecular formula of glucose is C6H12O6, which shows that each molecule of glucose contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.

Now, let's discuss how the molecular formula can be calculated if the molar mass is known after having an empirical formula:

Step 1: Determine the empirical formula.
- The empirical formula is usually determined experimentally or given in the problem.
- It represents the simplest ratio of atoms in the compound.

Step 2: Calculate the empirical formula mass.
- The empirical formula mass is the sum of the atomic masses of all the atoms in the empirical formula.
- It can be calculated using the periodic table.

Step 3: Determine the molar mass of the compound.
- The molar mass is the mass of one mole of the compound.
- It can be determined experimentally or given in the problem.

Step 4: Calculate the ratio between the molar mass and empirical formula mass.
- Divide the molar mass by the empirical formula mass to determine the ratio.
- This ratio represents the number of empirical formula units present in one mole of the compound.

Step 5: Multiply the subscripts in the empirical formula by the ratio obtained in step 4.
- This will give the molecular formula of the compound.

Therefore, the molecular formula can be calculated if the molar mass is known after having an empirical formula. The molar mass provides the necessary information to determine the ratio between the empirical formula units and the actual number of atoms in the compound, thus allowing the calculation of the molecular formula.