Chemistry · Some Basic Concepts Of Chemistry · NEET
Empirical formula = the simplest whole-number ratio of atoms. Molecular formula = the actual number of atoms in one molecule. Example: glucose molecular formula is C6H12O6, but its empirical formula is CH2O (divide all subscripts by 6). Both describe the same compound, but the molecular formula gives the real count, while the empirical formula only gives the ratio.
Because experiments (like percentage composition or combustion analysis) directly give you ratios of atoms, not exact counts. From percent mass you can only find the simplest ratio, which is the empirical formula. To get the molecular formula you also need the molar mass. So the empirical formula is the first step, and NEET often stops there.
Use n = (molar mass) / (empirical formula mass). Then Molecular formula = n x Empirical formula. Example: empirical formula CH2O has mass 30. If molar mass = 180, then n = 180/30 = 6, so molecular formula = C6H12O6. n is always a whole number.
Yes. When n = 1, they are identical. Examples: water H2O, carbon dioxide CO2, ammonia NH3, methane CH4. Here the simplest ratio is already the actual number of atoms, so no further dividing is possible.
Step 1: Take mass = percentage for each element. Step 2: Divide each by its atomic mass to get moles. Step 3: Divide all mole values by the smallest one. Step 4: Round to whole numbers to get the ratio. That ratio is the empirical formula. This is the exact method NEET tests almost every year.
Glucose C6H12O6 gives empirical formula CH2O. Benzene C6H6 gives empirical formula CH. Hydrogen peroxide H2O2 gives empirical formula HO. Just divide all subscripts by their greatest common factor.
An organic compound contains 78% (by wt.) carbon and the remaining percentage of hydrogen. The empirical formula of the compound is [at. wt.: C = 12, H = 1]
A compound X contains 32% of A, 20% of B and the remaining percentage of C. The empirical formula of X is (atomic masses: A = 64, B = 40, C = 32 u)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. By definition the empirical formula is the simplest whole-number ratio of atoms of each element in a compound. If you can still divide all subscripts by a common number, you have not reached the empirical formula yet.
Yes. The molecular formula gives the true number of atoms of each element present in one molecule of the compound, for example C6H12O6 for glucose.
Molecular formula = n x Empirical formula, where n = molar mass / empirical formula mass. n is always a whole number (1, 2, 3, and so on).
No. Ionic compounds such as NaCl exist as giant lattices, not separate molecules, so we write only the formula unit (which is basically the empirical formula). This is why we use formula mass, not molecular mass, for them.
NEET Chemistry almost every year asks you to find the empirical formula from percentage composition (as in 2021 and 2024). It is a quick, high-scoring calculation once you know the divide-by-smallest method.
