Chemistry · Some Basic Concepts Of Chemistry · NEET
Mass percent of an element = (mass of that element in one mole of the compound / molar mass of the compound) x 100. The 'mass of that element in one mole' means: (number of atoms of that element in the formula) x (its atomic mass). Do this for each element. This is the single most useful formula for this topic in NEET.
Molar mass of ethanol = 2(12.01) + 6(1.008) + 16.00 = 46.068 g/mol. There are 2 carbon atoms, so carbon mass = 2 x 12.01 = 24.02 g. Mass percent of C = (24.02 / 46.068) x 100 = 52.14%. This is the exact worked example given in NCERT Unit 1.
Yes. Since you are splitting the total mass (100%) among all the elements, the mass percents must add to 100% (small rounding differences are fine). NEET uses this shortcut: if a compound has only two elements and one is given as 78%, the other is simply 100 - 78 = 22%. You do not need to calculate it separately.
Percentage composition gives you the mass percent of each element. The empirical formula is the simplest whole-number ratio of atoms. You use percentage composition to FIND the empirical formula: assume 100 g of compound, so each percent becomes grams, then divide each mass by that element's atomic mass to get moles, then divide all mole values by the smallest one. NEET loves questions that give you percentages and ask for the empirical formula.
No. Percentage composition depends only on the formula and atomic masses, not on how big the sample is. 1 g or 1 kg of water is always 11.1% hydrogen and 88.9% oxygen. This is really the Law of Definite Proportions in action - the composition is fixed.
Use the standard values: H = 1, C = 12, N = 14, O = 16, S = 32, Cl = 35.5, unless the question gives you specific values (NEET often prints them, like 'at. wt.: C = 12, H = 1'). Always use the numbers the question gives - they may round differently than the ones you memorised.
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 the mass of each element written as a percent of the total mass of the compound. For example, water is about 11% hydrogen and 89% oxygen by mass. It shows how the total mass is shared among the elements.
Mass percent = (mass of the element in one mole of compound / molar mass of the compound) x 100. The mass of the element = (number of its atoms in the formula) x (its atomic mass).
Assume 100 g of the compound so each percent becomes grams. Divide each mass by that element's atomic mass to get moles. Then divide all mole numbers by the smallest value. The whole-number ratio you get is the empirical formula.
No. It only depends on the compound's formula and atomic masses. 1 gram and 1 kilogram of the same compound have exactly the same percentage composition.
Yes. NEET regularly asks you to find the empirical formula from given percentages (seen in 2021 and 2024). Learning the part-over-whole formula and the percent-to-mole conversion covers most of these questions.
