The vocabulary of this chapter forms one chain: relative masses lead to the mole, the mole links mass, particles and gas volume, formulae describe substances, and balanced equations and mole ratios turn all of it into stoichiometry.
The terms in The Mole Concept, Chemical Formula and Equation are best learned as one connected chain rather than a list of separate definitions. The chapter begins with a way of comparing the masses of atoms, uses that to build the mole, and then uses the mole to link mass, number of particles and the volume of a gas. From there it turns to the formulae that describe substances and the equations that describe reactions, ending with stoichiometry, where every earlier idea is put to work at once. Seeing these links is the fastest route to the precise wording the marking scheme rewards, because many of the marks in this chapter are won by defining a term exactly.
Relative masses: the starting point. Everything rests on the carbon-12 standard, the agreed reference against which atomic masses are compared. The relative atomic mass is the average mass of an atom compared with one-twelfth of a carbon-12 atom, and adding these values gives the relative molecular mass for a molecule or the relative formula mass for an ionic compound. The pair students most often confuse is relative molecular mass and relative formula mass: they are found in exactly the same way, but the first is used for molecules and the second for ionic compounds, which have no molecules. All of these are ratios with no unit, a point that matters as soon as molar mass appears.
The mole and its constants. The mole is the amount of substance that contains as many particles as there are atoms in 12 g of carbon-12, and the Avogadro constant is exactly how many particles that is. The molar mass is the mass of one mole in grams per mole, and its great convenience is that it is numerically equal to the relative atomic or molecular mass. Here two confusions arise. First, the mole is a huge number of particles, not a single particle, so it must not be muddled with the molecule. Second, the Avogadro constant is a fixed number of particles per mole, while Avogadro’s law is a separate statement about the volumes of gases; they share a name but describe different things.
Gases. Because of Avogadro’s law, one mole of any gas has the same molar volume under the same conditions. Students must keep two reference conditions apart: at room conditions the molar volume takes one value, and at standard temperature and pressure it takes another, so using the wrong value spoils an otherwise correct method. Molar volume is the gas counterpart of molar mass, giving a third route into the mole alongside mass and number of particles.
Formulae. A chemical formula is the umbrella term, and the empirical formula (the simplest ratio), the molecular formula (the actual atoms in a molecule) and the ionic formula (the ratio of ions in a neutral compound) are all kinds of it. The classic confusion is empirical versus molecular: the molecular formula is the empirical formula or a whole-number multiple of it, and you need the relative molecular mass to decide the factor. Two related terms complete the picture: water of crystallisation, which must be included in the relative formula mass of a hydrated salt, and percentage composition by mass, which turns a formula into the fraction of mass from each element.
Equations and stoichiometry. A balanced equation has equal numbers of each atom on both sides, achieved by changing coefficients and never subscripts. Its coefficients give the mole ratio, and applying that ratio, always in moles rather than grams, is the heart of stoichiometry. This is the destination the whole chapter leads to, because it draws together the mole, molar mass, molar volume, the balanced equation and the mole ratio in a single calculation.
Held together this way, the vocabulary of SPM Chemistry stops being a list and becomes a chain in which each term supports the next. Our teachers work through it in this order in our online one-to-one lessons, so that each definition reinforces the one before and the calculations that follow feel natural. Once the connections are clear, the definitions almost learn themselves, and the marks that come from precise wording follow.
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