Acids, bases and salts

Traps in Acids, bases and salts

• Ignores stoichiometric ratio
  Always balance the equation first, then use the coefficients as the mole ratio between the substances asked about.
• Concentration unit slips
  Concentration (mol dm⁻³) = moles ÷ volume in dm³. 1000 cm³ = 1 dm³, so divide cm³ by 1000.
• Confuses pH and pOH
  Use pH = −log[H⁺] when you are given [H⁺]. Only when you are given [OH⁻] do you find pOH first and then pH = 14 − pOH. Check: acids < 7, alkalis > 7.
• Drops the minus sign in pH = −log[H⁺]
  pH = −log₁₀[H⁺]. For [H⁺] = 1 × 10⁻ⁿ, pH = n. Ordinary acids and alkalis have pH between 0 and 14.
• pH misconceptions
  pH = −log₁₀[H⁺]. A tenfold change in [H⁺] changes pH by 1. Lower pH = more acidic.
• Mixes up a pair of similar chemical terms
  Keep a list of confusable pairs with one defining feature each and a hook. Underline the key word in the stem and check it against the definition, not the sound of the name.
• Picks a related substance or process with a different role
  Learn reagents and processes as one-line 'name → job' cards and contrast neighbours in a table (process | product | catalyst | conditions). Match the job in the stem, not just the topic.
• Reverses a trend, direction or effect
  State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
• Over-generalises a rule beyond where it holds
  Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
• Uses everyday or macroscopic thinking instead of particles
  Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
• Acid–base definition errors
  Brønsted–Lowry: acid = proton donor, base = proton acceptor. STRONG = fully ionised; CONCENTRATED = much solute per dm³. A dilute strong acid is still strong.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.
• Puts a substance or reaction in the wrong class
  Classify from the defining feature: functional group for organic series; bond type and structure for properties; reaction with acid AND alkali for amphoteric. Check each option against the feature, not the name.
• Miscounts atoms, electrons or steps
  After working, check by counting back: total electrons = atomic number; atoms in the formula match the subscripts; number of half-lives = time ÷ half-life.