Measurement of Matter — Practice Exercises: quick revision, 15 bites

Atoms Are the Building Blocks of All Matter

Everything you touch is atoms in disguise.

Matter is made of tiny particles called **atoms**. Atoms of different elements combine in fixed ratios to form **molecules**, and this ratio is what a chemical formula expresses.

Takeaway. Every chemical formula is a shorthand for how many atoms of each element combine in a molecule.

Valency: The Combining Power of an Atom

Valency decides who bonds with whom.

**Valency** is the number of electrons an atom gains, loses, or shares to achieve a stable (noble-gas) configuration.

Takeaway. Knowing valencies lets you write any chemical formula by cross-multiplying them.

Writing a Chemical Formula from Valencies

Cross the valencies, drop the signs.

Takeaway. Formula writing is mechanical: swap valencies, simplify, and you're done.

Relative Molecular Mass: Add the Atomic Masses

One addition solves most mass problems.

Takeaway. Molecular mass = sum of (atomic mass × subscript) for every atom in the formula.

The Mole: Chemistry's Counting Unit

One mole = 6.022 × 10²³ particles.

Takeaway. Moles link mass in grams to number of particles — the bridge in every stoichiometry problem.

Avogadro's Number in One Line

6.022 × 10²³ — memorize this number.

One mole of any substance contains **6.022 × 10²³** particles (atoms, molecules, or ions). This is Avogadro's number, denoted $N_A$.

Takeaway. Avogadro's number converts moles to particle count in one multiplication.

Don't Confuse Atomic Mass with Molecular Mass

One atom vs. whole molecule — not the same.

Takeaway. Molecular mass always accounts for every atom in the formula — subscripts matter.

Simplify Subscripts in Chemical Formulae

Mg₂O₂ is wrong — it's MgO.

Takeaway. Chemical formulae always use the simplest whole-number ratio of atoms.

Worked Example: Moles from Mass

36 g of water — how many moles?

Takeaway. Mass ÷ molar mass = moles. Practice this until it's automatic.

Worked Example: Particles from Moles

2 moles — how many molecules?

Takeaway. Moles × 6.022 × 10²³ = number of particles.

Atom vs. Molecule — Know the Difference

One particle or many bonded?

Takeaway. Atoms are single particles; molecules are groups of atoms bonded together.

Remember Common Valencies with a Mnemonic

HONClBrIF — the valency-1 club.

The diatomic elements **H, O, N, Cl, Br, I, F** all exist as molecules of two atoms — and their common valencies follow a simple pattern: H and halogens = 1, O = 2, N = 3.

Takeaway. Grouping elements by valency makes formula writing faster and less error-prone.

Exam Tip: Show Units and Steps

Marks hide in the working.

Takeaway. Step-by-step working with units earns partial marks even if the final answer slips.

Why Moles Matter in Real Life

Pharmacists count molecules, not grams.

Doctors prescribe medicines in moles because reactions happen particle-to-particle, not gram-to-gram. A 500 mg paracetamol tablet contains a precise number of molecules — calculated using the mole concept.

Takeaway. The mole concept is the reason chemists can predict exactly how much of a substance reacts.

Watch Out: Relative vs. Absolute Mass

u is not grams.

Takeaway. Keep relative mass (u) and molar mass (g/mol) in separate steps to avoid unit errors.