Chemistry · Some Basic Concepts Of Chemistry · NEET
Follow three steps. Step 1: Add the atomic masses of all atoms in the empirical formula to get the empirical formula mass. Step 2: Divide the molar mass by this empirical formula mass. This gives a whole number, called n. Step 3: Multiply every subscript in the empirical formula by n. Example: empirical formula CH2Cl has mass 12 + 2(1) + 35.5 = 49.5. If molar mass = 99, then n = 99/49.5 = 2, so molecular formula = C2H4Cl2.
n is how many empirical-formula units are joined together to make one real molecule. Because you cannot have half an atom, n must be a whole number like 1, 2, 3, or 4. If your division gives something like 1.98 or 2.03, round it to the nearest whole number (2). A value far from a whole number, like 1.5, usually means a calculation mistake or wrong data.
Empirical formula mass is the mass of the simplest ratio unit (like CH2Cl = 49.5). Molar mass is the mass of one mole of the real molecule (like C2H4Cl2 = 99). The molar mass is always n times the empirical formula mass. So molar mass / empirical formula mass = n.
When n = 1. This happens when the molar mass already equals the empirical formula mass. Examples include water H2O, carbon dioxide CO2, and ammonia NH3 — their simplest ratio is also the real molecule. In these cases you do not multiply anything.
First find the empirical formula from the percent data. Divide each element's percent by its atomic mass to get moles, then divide all values by the smallest one to get the simplest whole number ratio. That ratio is the empirical formula. Only after that do you use the molar mass to find n and the molecular formula.
Yes. The empirical formula alone only gives the ratio of atoms. Different molecules like CH2O, C2H4O2, and C6H12O6 all have the same empirical formula CH2O. Only the molar mass tells you which one it is, because it fixes the value of n.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Molecular formula = n x empirical formula, where n = molar mass / empirical formula mass. Round n to the nearest whole number.
No. n must be a whole number because a molecule contains whole atoms. A value like 1.5 usually signals a data or rounding error; recheck your empirical formula mass.
CH2O is the empirical formula. Glucose has molar mass 180, empirical mass 30, so n = 6, and the molecular formula is C6H12O6.
They all have carbon, hydrogen and oxygen in the ratio 1:2:1, which is CH2O. Their molar masses differ, so their n values (1, 2, 6) differ, giving different molecular formulas.
NEET often gives percent composition plus molar mass and asks for the molecular formula. Knowing the n = molar mass / empirical mass step lets you solve these quickly without guessing.
