Chemistry: Moles, Reactions, Equilibrium, and Structure
Chemistry on this exam tests quantitative reasoning about particles: how much reacts, what limits the reaction, how fast and how far it proceeds, and how structure explains properties. Expect mole calculations, periodic trends, acid-base and redox ideas, and equilibrium reasoning.
Core rules
- The mole connects mass, particles, and gas volume: $n=m/M$, $N=nN_A$, and at standard conditions one mole of ideal gas occupies about 22.4 L.
- Balance equations by atoms and charge. Use the balanced coefficients to convert moles of one species to moles of another.
- Identify the limiting reagent by dividing each reactant's available moles by its coefficient; the smallest quotient limits product formation.
- Periodic trends: atomic radius generally decreases across a period and increases down a group; ionization energy generally increases across a period and decreases down a group.
- For equilibrium, $K$ is constant at a given temperature. Changing concentration or pressure shifts the position but not $K$; changing temperature can change $K$ itself.
- pH $=-\log[\mathrm{H^+}]$ for dilute aqueous solutions at 25 °C, where pH + pOH = 14.
Worked example
10.0 g of calcium carbonate reacts with 0.200 L of 1.00 M hydrochloric acid: $\mathrm{CaCO_3+2HCl\rightarrow CaCl_2+CO_2+H_2O}$. Moles of $\mathrm{CaCO_3}=10.0/100.1\approx0.0999$ mol. Moles of HCl $=0.200$ mol. The acid needs $2(0.0999)\approx0.200$ mol, so the reactants are nearly stoichiometric and $\mathrm{CaCO_3}$ is very slightly limiting. Carbon dioxide produced is about 0.0999 mol, or $0.0999\times22.4\approx2.24$ L at standard conditions.
Traps that cost marks
- Choosing the limiting reagent by smaller mass rather than by mole ratio.
- Confusing pH with pOH, or forgetting that strong acids dissociate completely while weak acids do not.
- Writing equilibrium expressions with pure solids or liquids included.
- Ignoring state symbols or charge balance in redox half-reactions.
- Rounding intermediate mole values too early.