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Interconverting Mass, Moles and Number of Particles

Chemistry · Some Basic Concepts Of Chemistry · NEET

To go between mass, moles and number of particles, always pass through moles first. Moles = mass divided by molar mass; particles = moles multiplied by Avogadro's number (6.022 x 10^23). Memory hook: "Mass over molar mass gives moles; moles times NA gives particles" - moles is the middle station every train must stop at.
The Mole Bridge: Mass to ParticlesMASSgrams (g)MOLESamount (mol)PARTICLESatoms/moleculesdivide bymolar massx molar massmultiply byNA (6.022e23)divide by NAFor atoms in a compound: particles x (atoms per formula unit)Every conversion must stop at MOLES in the middle.
The mole bridge: mass connects to particles only through moles. Divide mass by molar mass to get moles, then multiply moles by Avogadro's number to get particles (reverse the arrows to go backward).

Your doubts, answered

How do I convert grams to number of particles? What is the order of steps?

You cannot jump straight from grams to particles. Always go grams -> moles -> particles. Step 1: moles = mass (g) / molar mass (g/mol). Step 2: particles = moles x 6.022 x 10^23. Example: 5.4 g of urea (molar mass 60). Moles = 5.4/60 = 0.09 mol. Molecules = 0.09 x 6.022 x 10^23 = 5.42 x 10^22 molecules. This exact chain appeared in NEET 2026.

When do I multiply by Avogadro's number and when do I divide?

Simple rule: moles -> particles means you MULTIPLY by 6.022 x 10^23 (one mole is a huge number of particles, so the count goes up). Particles -> moles means you DIVIDE by 6.022 x 10^23 (you are packing many particles into fewer moles). If your answer for number of atoms is smaller than the number of moles, you divided by mistake.

How do I find the number of ATOMS, not molecules?

First find molecules using moles x NA. Then multiply by the number of atoms in one molecule (from the formula). Example: H2O has 3 atoms per molecule. 1 mole of water = 6.022 x 10^23 molecules = 3 x 6.022 x 10^23 atoms. For urea NH2CONH2 there are 4 H atoms per molecule, so H atoms = moles x 4 x NA. Forgetting this extra multiply is the most common NEET slip.

What molar mass do I divide by - the whole compound or one element?

Divide by the molar mass of the whole substance you weighed. For O2 gas use 32 g/mol (not 16). For NaCl use 58.5 g/mol. Only after you have moles of the compound do you break it down into individual atoms using the formula. Weighing O2 and dividing by 16 is a classic trap.

How do I know the number of atoms per formula unit?

Count every atom written in the chemical formula. Na2CO3 has 2 Na + 1 C + 3 O = 6 atoms per formula unit. CO2 has 3 atoms. NaOH has 3 atoms. He (monatomic) has 1. Total atoms = moles x (atoms per formula unit) x NA. This is exactly how NEET 2025 tested it.

For a gas, can I use 22.4 L to find moles instead of mass?

Yes, but only at STP. For a gas at STP, moles = volume (L) / 22.4 L (older value) or / 22.7 L (new NCERT value). Then multiply by NA for particles. So a gas gives you a third road into moles: mass road (mass/molar mass), OR volume road (volume/molar volume). All roads meet at moles.

⚠️ The NEET trap
✗ To find atoms in 1 g of O2, students do (1/16) x NA = 3.76 x 10^22 atoms, using atomic mass 16.
✓ O2 is a molecule with molar mass 32. Moles = 1/32. Molecules = (1/32) x NA. Atoms = (1/32) x 2 x NA = 3.76 x 10^22 atoms. The 2 (atoms per molecule) cancels the mistake of using 16, so the number looks right - but the reasoning is wrong and it fails for other cases.
🧠 Weigh the whole molecule, then split into atoms. Never divide mass by a single atom's mass for a diatomic gas.

Real NEET questions

NEET 2026

The number of hydrogen atoms present in 5.4 g of urea is (molar mass of urea = 60 g/mol; NA = 6.022 x 10^23 /mol)

A · 1.084 x 10^23
B · 1.084 x 10^22
C · 2.168 x 10^22
D · 2.168 x 10^23 ✓
Solution: Urea (NH2CONH2) has 4 H atoms per molecule. Moles of urea = 5.4/60 = 0.09 mol. H atoms = 0.09 x 4 x 6.022 x 10^23 = 2.168 x 10^23. This is the full mass -> moles -> particles -> atoms chain.
NEET 2020

Which one of the following has the maximum number of atoms? (A) 1 g of O2 [O=16] (B) 1 g of Li [Li=7] (C) 1 g of Ag [Ag=108] (D) 1 g of Mg [Mg=24]

A · 1 g of O2
B · 1 g of Li ✓
C · 1 g of Ag
D · 1 g of Mg
Solution: Atoms = (mass/molar mass) x atoms-per-unit x NA. For a fixed 1 g, the smallest mass per atom gives the most atoms. Li (7 g/mol, monatomic) has the lowest value, so 1 g of Li has the maximum number of atoms.
NEET 2025

Choose the ones with an equal number of atoms: A) 212 g Na2CO3 [106] B) 248 g Na2O [62] C) 240 g NaOH [40] D) 12 g H2 [2] E) 220 g CO2 [44]

A · B, C and D only
B · B, D and E only
C · A, B and C only
D · A, B and D only ✓
Solution: Total atoms = moles x atoms per formula unit (in NA units). A: (212/106)=2 mol x 6 = 12. B: (248/62)=4 mol x 3 = 12. C: (240/40)=6 mol x 3 = 18. D: (12/2)=6 mol x 2 = 12. E: (220/44)=5 mol x 3 = 15. A, B and D each give 12 NA atoms.

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Frequently asked

What is the value of Avogadro's number?

Avogadro's number (NA) is 6.022 x 10^23. It is the number of particles (atoms, molecules or ions) in exactly one mole of any substance. NCERT writes it fully as about 602213670000000000000000.

What is the one formula I must remember for all conversions?

Moles = mass / molar mass, and particles = moles x NA. Combine them: number of particles = (mass / molar mass) x NA. For atoms in a compound, multiply again by atoms per formula unit.

Why must moles always be the middle step?

Mass and particle count are not directly linked - they connect only through the amount of substance, which is measured in moles. Moles is the bridge that molar mass and Avogadro's number both plug into, so every conversion must pass through it.

How is molar mass different from molecular mass?

They are numerically equal but have different units and meaning. Molecular mass is in u (for one molecule); molar mass is the same number in g/mol (for one mole). Water is 18 u per molecule and 18 g/mol per mole.

How do I find moles of a gas if I only know its volume?

At STP, moles = volume in litres / 22.4 L (or 22.7 L in the newest NCERT). Then multiply by NA for the number of molecules. This gives a volume route into moles for gases.

Related concepts:
Molar Volume of a Gas at STP (22.4 L / 22.7 L)Which Sample Has the Maximum Number of Atoms or MoleculesGram Atomic Mass and Gram Molecular MassMolecular Mass vs Formula Mass (When to Use Which)Mole-to-Mole Relationships in Chemical Reactions
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Some Basic Concepts Of Chemistry PYQs