Chemistry · Some Basic Concepts Of Chemistry · NEET
They have the SAME number but DIFFERENT units. Molecular mass is measured in u (atomic mass units) — for example, water (H2O) has molecular mass 18 u. Molar mass is that same value but in grams per mole (g/mol) — so 1 mole of water weighs 18 g/mol. Rule to remember: molecular mass = mass of one molecule (in u); molar mass = mass of one mole of molecules (in g). The numbers match; only the unit changes.
Gram atomic mass is just the molar mass of an ELEMENT taken as single atoms — it is the atomic mass of that element expressed in grams. Example: atomic mass of carbon = 12 u, so gram atomic mass of carbon = 12 g. Molar mass is the general word — it works for atoms, molecules, or ions. So gram atomic mass is a special case of molar mass used only for atoms of elements.
One atom is too tiny to weigh. Instead we count a huge fixed number of atoms — one mole = 6.022 x 10^23 atoms (Avogadro number). When you take that many atoms, the total mass in grams comes out equal to the atomic mass number. This is on purpose: the mole was defined so that the atomic mass in u becomes the molar mass in grams. That is why molar mass is always in g/mol.
It depends on what you mean. An oxygen ATOM (O) has atomic mass 16 u, so gram atomic mass = 16 g/mol. But oxygen GAS is O2 (two atoms joined), so its molar mass = 2 x 16 = 32 g/mol. In NEET questions, if they say 'oxygen gas' or write O2, use 32 g/mol. If they say 'oxygen atom' or O, use 16 g/mol. Read the formula carefully.
Both appear because STP was redefined. OLD STP (0 C, 1 atm) gives molar volume = 22.4 L/mol. NEW STP used by the latest NCERT (0 C, 1 bar) gives 22.7 L/mol. NEET has used both — recent papers (2024) use 22.7 L. In an exam, use the value that matches the numbers given in the question. If the question uses 22.4, follow it; if it uses 22.71, follow that.
Add up the gram atomic masses of every atom in the formula. Example, CO2: carbon = 12, oxygen = 16 x 2 = 32, total = 12 + 32 = 44 g/mol. For CaCO3: Ca = 40, C = 12, O = 16 x 3 = 48, total = 100 g/mol. Just multiply each atom's atomic mass by how many of that atom there are, then add everything.
The highest number of helium atoms is present in
In which of the following is the number of water molecules maximum?
Which one of the following has the maximum number of atoms? (1 g each; atomic masses 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.
Molar mass is how many grams one mole (6.022 x 10^23 particles) of a substance weighs. Its unit is g/mol. For example, 1 mole of water weighs 18 g, so the molar mass of water is 18 g/mol.
Gram atomic mass is the atomic mass of an element written in grams. It is the molar mass of the element as single atoms. Example: sodium has atomic mass 23 u, so its gram atomic mass is 23 g/mol.
Molar volume is the volume that 1 mole of any gas takes up at STP. At old STP (0 C, 1 atm) it is 22.4 L/mol. At new STP (0 C, 1 bar) used by current NCERT it is 22.7 L/mol. It is the same for all ideal gases.
By Avogadro's law, equal volumes of gases at the same temperature and pressure hold equal numbers of molecules. So 1 mole of any gas (same molecule count) fills the same volume, no matter which gas it is. Gas identity does not change the volume.
Yes. Molar mass is always written with the unit gram per mole (g/mol). This is what makes it different from atomic or molecular mass, which use u and have no grams.
