Everything from the 6 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
In an ionic compound the total positive charge equals the total negative charge, so Al3+ and O2− combine 2 : 3 as Al2O3. A balanced symbol equation has the same number of each type of atom on both sides. Balance it by changing only the big numbers in front of formulae, never the small numbers inside them. State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Never balance an equation by changing a small number inside a formula. Writing H2O2 to make H2 + O2 → H2O balance turns water into hydrogen peroxide, a different substance. Change only the numbers in front: 2H2 + O2 → 2H2O.
Relative atomic mass Ar is the average mass of the isotopes of an element compared with 1/12 of the mass of an atom of carbon-12. Relative molecular mass Mr is the sum of the Ar values of all the atoms in a formula; the same sum is used for ionic compounds (relative formula mass). Relative masses are ratios, so they have no units.
Count every atom of the element. Ammonium nitrate, NH4NO3, has one N in the ammonium ion and one in the nitrate ion, so the nitrogen mass is 28, not 14. Using 14 gives 17.5%, exactly half the right answer.
One mole of a substance contains 6.02 × 1023 particles (the Avogadro constant). The mass of one mole in grams equals its Ar or Mr, so moles = mass ÷ Mr. One mole of any gas occupies 24 dm3 at room temperature and pressure (r.t.p.), so moles of gas = volume in dm3 ÷ 24, or volume in cm3 ÷ 24 000.
Read whether the question wants molecules or atoms. 9 g of water is 3.01 × 1023 molecules but 9.03 × 1023 atoms, because each H2O molecule contains three atoms.
The 24 dm3 rule is for gases only, at r.t.p. Never use it for a solid, a liquid or a solution. And check the unit: dividing a volume in cm3 by 24 gives an answer 1000 times too big.
Use three steps: (1) change the mass (or gas volume) you are given into moles; (2) use the numbers in the balanced equation to find the moles of the substance you want; (3) change those moles back into a mass or volume. The limiting reactant is the one that is completely used up; it fixes the amount of product. Percentage yield = actual mass ÷ expected mass × 100. Percentage purity = mass of pure substance ÷ mass of impure sample × 100.
The 2 in front of 2MgO belongs in the mole ratio, not in the Mr. Use Mr(MgO) = 40, not 80. A student who calculates 0.2 mol × 80 gets 16 g, double the correct 8.0 g.
Equal numbers of moles do not mean nothing is left over. Here there are 0.1 mol of each, but the equation needs twice as much HCl as Mg, so HCl runs out first. Always compare the moles you have with the ratio in the equation.
Concentration can be given in g/dm3 or in mol/dm3. Moles of solute = concentration in mol/dm3 × volume in dm3, and to change cm3 into dm3 you divide by 1000. In a titration you find the moles of the solution you know completely, use the equation's ratio, and then divide by the volume of the other solution in dm3.
Burettes and pipettes measure in cm3, but concentrations are per dm3. Forgetting to divide by 1000 makes 25 cm3 of 0.2 mol/dm3 look like 5 mol instead of 0.005 mol.
Do not skip the ratio. Treating the sulfuric acid titration as 1 : 1 gives 0.0025 ÷ 0.0125 = 0.20 mol/dm3, double the right answer. Write the balanced equation first and read the ratio from it.
The empirical formula is the simplest whole-number ratio of the different atoms or ions in a compound. The molecular formula is the number and type of each atom in one molecule. To find the empirical formula, divide each mass (or percentage) by Ar, then divide by the smallest answer. To find the molecular formula, divide Mr by the empirical formula mass and multiply the empirical formula by the answer.
Never round 1 : 1.5 to 1 : 2 (or to 1 : 1). A ratio ending in .5 means you must double every number. Rounding the iron oxide ratio gives FeO2, which is wrong; doubling gives Fe2O3. And never divide the masses themselves: 2.4 g Mg to 1.6 g O is a 1 : 1 ratio of atoms, not 3 : 2.