Chemistry · Some Basic Concepts Of Chemistry · NEET
A mole is a NUMBER, not a mass. It is a fixed count of particles, exactly like the word 'dozen' is a fixed count of 12. One mole = 6.022 x 10^23 particles. It does not matter what the particles are - atoms, molecules, ions or electrons. So '1 mole of oxygen atoms' means 6.022 x 10^23 oxygen atoms, and '1 mole of cars' would mean 6.022 x 10^23 cars. The mass changes with the substance, but the number of particles in one mole is always the same.
Avogadro's number (N_A) is the number of particles in exactly one mole: N_A = 6.022 x 10^23 per mole. This value is not random. It was fixed so that one mole of carbon-12 atoms weighs exactly 12 grams. Scientists measured the mass of a single carbon-12 atom (1.992648 x 10^-23 g) and divided 12 g by it, which gives 6.022 x 10^23. So Avogadro's number links the tiny world of atoms to grams we can weigh in a lab.
They are connected but not the same word. 'Mole' is the UNIT (the name of the packet). 'Avogadro's number' is the SIZE of that packet (6.022 x 10^23). Compare it with 'dozen' and '12': dozen is the unit, 12 is the count. So 1 mole = Avogadro's number of particles = 6.022 x 10^23 particles.
Atoms are extremely small and extremely many. Even a tiny 18 mL of water holds about 6 x 10^23 molecules. You could never count them one by one. So chemists group them into big fixed packets called moles, just like a shopkeeper counts eggs in dozens instead of one by one. This makes calculations short and easy, and it is important for NEET numericals where you convert grams to particles fast.
Use one simple rule: Number of particles = moles (n) x N_A, where N_A = 6.022 x 10^23. Example: 2 moles of water = 2 x 6.022 x 10^23 = 1.2044 x 10^24 water molecules. To go from mass to particles first find moles (n = given mass / molar mass), then multiply by N_A. For total ATOMS, also multiply by the number of atoms in one formula unit (water has 3 atoms, so multiply again by 3).
Both contain 6.022 x 10^23 units, but the unit is different. 1 mole of O atoms = 6.022 x 10^23 oxygen ATOMS. 1 mole of O2 molecules = 6.022 x 10^23 oxygen MOLECULES, and since each molecule has 2 atoms, that is 2 x 6.022 x 10^23 = 1.2044 x 10^24 atoms. NEET traps you here, so always read carefully: does the question ask for molecules or atoms?
The number of hydrogen atoms present in 5.4 g of urea is (molar mass of urea = 60 g/mol; N_A = 6.022 x 10^23 /mol)
The highest number of helium atoms is present in
Which one of the following has the maximum number of atoms?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 mole = 6.022 x 10^23 particles (atoms, molecules, ions or electrons). This count is Avogadro's number. For example, 1 mole of water = 6.022 x 10^23 water molecules.
Amedeo Avogadro proposed in 1811 that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. Avogadro's number (6.022 x 10^23) is named in his honour, though he did not calculate the number himself.
No. Avogadro's number is a COUNT (6.022 x 10^23 particles per mole). Molar mass is a MASS (grams per mole). They work together: molar mass in grams contains Avogadro's number of particles. That connection is covered in the next topic, molar mass and gram atomic mass.
Water (H2O) has 3 atoms per molecule. Molecules in 2 mol = 2 x 6.022 x 10^23. Total atoms = 2 x 6.022 x 10^23 x 3 = 3.6132 x 10^24 atoms. Always multiply by atoms per molecule when the question asks for atoms.
Almost every stoichiometry, concentration and empirical formula question needs moles. If you can quickly convert grams to moles and moles to particles using n = mass/molar mass and particles = n x N_A, you solve these NEET numericals in seconds.
