Chemistry · Some Basic Concepts Of Chemistry · NEET
Atomic mass is just a number written in u (amu). For example, the atomic mass of oxygen is 16 u. Gram atomic mass takes that same number and writes it in grams: 16 g. So atomic mass = 16 u (mass of ONE atom on the u scale), while gram atomic mass = 16 g (mass of ONE MOLE = 6.022 x 10^23 atoms). The number is the same; only the unit and the meaning change.
Yes, for a single element they are the same value. NCERT says the molar mass in grams is numerically equal to the atomic mass in u. So the gram atomic mass of sodium (23 g) is exactly its molar mass (23 g/mol). The word 'molar mass' is the modern term used in NEET; 'gram atomic mass' is the older name for the same idea when we talk about one element.
Water is H2O. Add the atomic masses: 2 x 1 (H) + 16 (O) = 18 u. That is the molecular mass. Now write it in grams: 18 g. So the gram molecular mass of water is 18 g, and this is the mass of 1 mole of water molecules (6.022 x 10^23 molecules).
Look up the atomic mass in the periodic table (in u), then write that same number with the unit grams. Carbon = 12 u, so gram atomic mass = 12 g. Iron = 56 u, so gram atomic mass = 56 g. No calculation is needed - just copy the number and change 'u' to 'g'.
Use gram atomic mass when the substance exists as single atoms (metals like Na, Fe, or noble gases like He, Ar). Use gram molecular mass when the substance is made of molecules (H2, O2, CO2, H2O, glucose). Example: for O2 gas use gram molecular mass 32 g, but for a single O atom use gram atomic mass 16 g. This is a very common NEET trap.
Atoms are far too tiny to weigh one at a time. By taking the atomic mass in grams, we get a weighable amount that contains exactly 1 mole of atoms. This lets us count particles just by weighing on a balance, which is the base of every mole and stoichiometry calculation in NEET.
Suppose the elements X and Y combine to form two compounds XY2 and X3Y2. When 0.1 mole of XY2 weighs 10 g and 0.05 mole of X3Y2 weighs 9 g, the atomic weights of X and Y respectively are
The weight (g) of two moles of the organic compound obtained by heating sodium ethanoate with sodium hydroxide in the presence of calcium oxide is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 gram atom means 1 mole of atoms of an element, and its mass equals the gram atomic mass. For example, 1 gram atom of sodium = 23 g = 6.022 x 10^23 sodium atoms.
1 gram molecule means 1 mole of molecules, and its mass equals the gram molecular mass. For example, 1 gram molecule of CO2 = 44 g = 6.022 x 10^23 CO2 molecules.
Yes, its unit is grams (g). When we say 'per mole' we write it as g/mol, which is the molar mass form used in NEET calculations.
Exactly Avogadro's number, 6.022 x 10^23 atoms. That is the whole point - the gram atomic mass is a weighable amount holding one mole of atoms.
Yes. NCERT states the molar mass in grams is numerically equal to the atomic, molecular, or formula mass in u. So 18 u molecular mass of water means 18 g/mol molar mass.
