Chemistry · Some Basic Concepts Of Chemistry · NEET
Every CO2 molecule has exactly one carbon atom. So mass of C = mass of CO2 x (12/44), because the molar mass of CO2 is 44 g and carbon is 12 g of that. Example: 0.88 g of CO2 contains 0.88 x (12/44) = 0.24 g of carbon. This is the key first move in almost every NEET combustion problem.
Every H2O molecule has two hydrogen atoms. So mass of H = mass of H2O x (2/18), because water's molar mass is 18 g and the two H atoms weigh 2 g. Example: 0.36 g of H2O contains 0.36 x (2/18) = 0.04 g of hydrogen. Do NOT use 1/18 by accident there are two hydrogens in water.
Add up the mass of C and the mass of H you found. If this total is LESS than the mass of the original sample, the missing mass is oxygen. Mass of O = sample mass minus (mass C + mass H). You cannot get oxygen straight from CO2 or H2O, because that oxygen came from the air you burned it in, not from the compound. This is the single biggest trap in NEET.
The empirical formula is the simplest whole-number ratio of atoms. Dividing every mole value by the smallest one forces the smallest element to become 1, and shows how many times bigger the others are. If the answer comes out like 1 : 2.5, multiply everything by a number (here 2) to clear the decimal, giving 2 : 5.
No. The empirical formula is only the simplest ratio (like CH2). The molecular formula is the real number of atoms in one molecule (like C4H8). To get the molecular formula you also need the molar mass: divide molar mass by empirical formula mass to get a whole number n, then multiply the empirical formula by n.
Convert each mass to moles by dividing by that element's atomic mass. Then divide all mole values by the smallest. Round to whole numbers (or clear decimals) to get the atom ratio. Write the symbols with those numbers that is your empirical formula.
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.
It is burning a known mass of an organic compound in plenty of oxygen so all carbon turns into CO2 and all hydrogen turns into H2O. You then weigh the CO2 and H2O to work out how much C and H were in the sample.
Multiply CO2 mass by 12/44 to get carbon. Multiply H2O mass by 2/18 (which is 1/9) to get hydrogen. These fractions come from the molar masses: CO2 = 44 g with 12 g carbon, H2O = 18 g with 2 g hydrogen.
No, this CO2/H2O method only gives carbon and hydrogen directly. Other elements like N, S, or halogens need separate measurements. Oxygen is found by difference.
NEET regularly asks empirical-formula questions from Some Basic Concepts of Chemistry. The mole-ratio skill and the 'oxygen by difference' trap appear often, so mastering this method scores easy marks.
