Amount of Substance

• The mole is chemistry's "counting unit" — it links mass, particles, gas volume, and solution concentration all together.
• Concentration can be measured as mass concentration (g dm⁻³) or molar concentration (mol dm⁻³) — and at very low levels, in parts per million (ppm).
• Reacting mass calculations use the mole ratio from a balanced equation to find unknown masses of reactants or products.
• You can even work backwards — using masses from an experiment to figure out the balanced equation itself.
• Reacting volume calculations apply to gases: at r.t.p. one mole of any gas occupies 24.0 dm³; at s.t.p. it's 22.4 dm³.
• The ideal gas equation PV = nRT handles gases at any temperature and pressure, not just standard conditions.
• Percentage yield compares what you actually got to the maximum theoretically possible.
• Atom economy tells you how much of your reactant mass actually ends up in the product you want (vs. wasted as by-products).

Atoms and molecules are absurdly small and absurdly numerous — you can't count them one by one. So chemists invented the mole: a "chemist's dozen" that lets us talk about huge numbers of particles using ordinary numbers we can calculate with.

Because "amount of substance" is really a blanket term for almost any calculation involving moles, there are several routes into finding n (moles) depending on what information you're given:

"Concentration" just means how much stuff is packed into a given volume of solution — but there are two ways to express "how much stuff":

• Mass concentration — measured in g dm⁻³ (or g cm⁻³, kg m⁻³, mg/ml). This is literally grams of solute per volume of solution — no chemistry needed, just division.
• Molar concentration — measured in mol dm⁻³. This tells you how many moles of solute are packed into the solution, which is far more chemically useful because moles react in fixed ratios, not grams.