When can the empirical formula be converted to the molecular formula?

When can the empirical formula be converted to the molecular formula?

• A. Never
• B. If the compound is ionic
• C. If the molar mass of the compound is known
• D. Always

Answer: (C)

The key idea is that the molecular formula is an integer multiple of the empirical formula. You determine that multiple by using the molar masses. Specifically, the number n that relates them is n = (molar mass of the compound) / (molar mass of the empirical formula). Once you have n, you multiply the empirical formula by n to get the molecular formula. So you can convert only if you know the compound’s molar mass. For example, if the empirical formula is CH2O (molar mass about 30 g/mol) and the actual molar mass is 180 g/mol, then n ≈ 180/30 = 6, giving C6H12O6. Without the molar mass, you can’t determine n, so the molecular formula can’t be found from the empirical formula alone.

The key idea is that the molecular formula is an integer multiple of the empirical formula. You determine that multiple by using the molar masses. Specifically, the number n that relates them is n = (molar mass of the compound) / (molar mass of the empirical formula). Once you have n, you multiply the empirical formula by n to get the molecular formula. So you can convert only if you know the compound’s molar mass.

For example, if the empirical formula is CH2O (molar mass about 30 g/mol) and the actual molar mass is 180 g/mol, then n ≈ 180/30 = 6, giving C6H12O6. Without the molar mass, you can’t determine n, so the molecular formula can’t be found from the empirical formula alone.