Chemistry · Some Basic Concepts Of Chemistry · NEET
Follow 3 steps every time. Step 1: moles = given mass ÷ molar mass. Step 2: number of molecules (or formula units) = moles × 6.022 × 10²³. Step 3: number of atoms = number of molecules × (atoms in one molecule). Example: 5.4 g of urea (molar mass 60). Moles = 5.4/60 = 0.09. Urea NH2CONH2 has 4 H atoms per molecule, so H atoms = 0.09 × 4 × 6.022×10²³ = 2.168×10²³. Never skip the moles step.
For a gas at STP, use the molar volume as a bridge instead of molar mass. Moles = volume ÷ 22.7 L (the current NCERT STP molar volume; older books use 22.4 L). Then molecules = moles × 6.022×10²³. Example: 2.271 L of helium at STP = 2.271/22.7 ≈ 0.1 mol = 0.1 × 6.022×10²³ atoms. Watch which STP value the question uses — NEET now mostly uses 22.7 L, so read the numbers in the options.
For a fixed mass, smaller molar mass means more atoms. Atoms = (mass/molar mass) × NA × (atoms per unit). So compare molar mass per atom. In NEET 2020, 1 g of Li (7 g/mol, monatomic) beats 1 g of O2, Ag, and Mg because Li has the lowest mass per atom. Careful: O2 has 2 atoms per molecule, so multiply by 2 — but even then Li still wins because its molar mass is very small.
Molecules (or formula units) = moles × NA. Atoms = molecules × (atoms in one molecule). For water H2O, 1 mole = 6.022×10²³ molecules but 3 × 6.022×10²³ atoms (2 H + 1 O). For Na2CO3, one formula unit has 6 atoms (2 Na + 1 C + 3 O). NEET loves to ask 'number of atoms', so always check how many atoms sit inside one molecule or formula unit before you finish.
Because they are different amounts of moles. Convert each to moles first. 18 mL water = 18 g (density 1 g/mL) = 18/18 = 1 mol. 0.18 g = 0.18/18 = 0.01 mol. 10⁻³ mol = 0.001 mol. 0.00224 L vapour at STP = 0.00224/22.4 = 10⁻⁴ mol. The biggest mole value has the most molecules, so 18 mL wins (NEET 2018). The lesson: never compare grams, mL, and litres directly — turn everything into moles.
Old NCERT and old NEET used 22.4 L at 273 K and 1 atm. New NCERT changed STP to 273.15 K and 1 bar, giving 22.7 L (written as 22.71098 L). NEET 2024 used 22.71098 L in a helium question. Tip: look at the numbers in the options. If the answer needs 22.7 to come out clean, use 22.7; if it needs 22.4, use 22.4.
The number of hydrogen atoms present in 5.4 g of urea is (molar mass of urea = 60 g mol⁻¹; NA = 6.022×10²³ mol⁻¹)
The highest number of helium atoms is present in
Which one of the following has the maximum number of atoms? (at. mass O = 16, Li = 7, Ag = 108, Mg = 24)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Number of atoms = (given mass ÷ molar mass) × 6.022×10²³ × (number of atoms in one molecule or formula unit). Always find moles first, then use Avogadro's number.
At STP, moles = volume ÷ 22.7 L (older papers use 22.4 L). Then molecules = moles × 6.022×10²³. Check the option numbers to see which STP value the question expects.
Yes. 6.022×10²³ is the number of particles in one mole, whether those particles are atoms, molecules, or ions. You then multiply by atoms-per-molecule if the question asks for atoms.
Because it tests one clean skill: converting different quantities (grams, moles, litres) into moles and comparing. It appeared in 2018, 2020, 2024, and 2025, so it is high-yield. Practice converting everything to moles fast.
Next is percentage composition, where you use these mole ideas to find the percent by mass of each element in a compound. It builds directly on counting atoms in a molecule.
