Chemistry · Some Basic Concepts Of Chemistry · NEET
Molecular mass is the mass of ONE molecule. But solid NaCl has no single molecule. As NCERT says, in the solid state sodium chloride does not exist as a single entity. So we cannot point to one NaCl molecule to weigh. We use formula mass instead, which just adds the atomic masses shown in the formula: 23 + 35.5 = 58.5 u.
No. NaCl is an ionic compound. It is made of Na+ ions and Cl- ions arranged in a repeating 3D lattice. One Na+ is surrounded by six Cl- ions, and each Cl- is surrounded by six Na+ ions. There is no separate NaCl unit sitting alone, so there is no molecule.
Use molecular mass for substances that exist as real molecules, like H2O, CO2, glucose, and O2. Use formula mass for ionic compounds that form lattices, like NaCl, KCl, CaO, and MgCl2. Simple test: if it is an ionic compound (metal + non-metal in a lattice), use formula mass.
Add the atomic mass of each atom in the formula. Na = 23 u and Cl = 35.5 u. So formula mass of NaCl = 23 + 35.5 = 58.5 u. The steps are exactly the same as molecular mass; only the name changes because there is no true molecule.
NEET can ask a direct concept question like: why do we use formula mass for NaCl? The correct reason is that NaCl does not exist as discrete molecules; it is an ionic solid with a lattice. If you pick 'because NaCl is covalent' or 'because Na and Cl share electrons', it is wrong. The name reflects the true structure, so learn the reason, not just the number.
They share the same number. Formula mass of NaCl is 58.5 u (mass of one formula unit). The molar mass is 58.5 g/mol (mass of one mole of formula units). One is in u for a single unit; the other is in grams per mole for Avogadro number of units.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
58.5 u. It is found by adding the atomic mass of sodium (23 u) and chlorine (35.5 u): 23 + 35.5 = 58.5 u.
Because NaCl has no single molecule. It is an ionic lattice of Na+ and Cl- ions. We add the masses in the formula unit, so it is called formula mass.
Yes. Ionic compounds like NaCl, KCl, CaO, and MgCl2 form lattices with no discrete molecules, so they all use formula mass instead of molecular mass.
Yes, the calculation is identical: add the atomic masses of all atoms in the formula. Only the name differs, based on whether real molecules exist.
