Stoichiometry

Traps in Stoichiometry

• Counts atoms instead of masses
  Multiply each atom count by its relative atomic mass first, then add. For empirical formulae, divide each percentage by its Ar to get moles before taking the ratio.
• Molar mass calculation errors
  Multiply each atom's Ar by the number of that atom, including everything inside brackets times the outside subscript: Ca(OH)₂ = 40 + 2(16 + 1) = 74.
• 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.
• Ignores or misapplies a subscript or bracket
  A subscript multiplies only the atom before it; a subscript after a bracket multiplies everything inside. Expand the formula into a full atom list before adding masses.
• Inverts n = m/M
  Moles = mass ÷ molar mass. Check units: g ÷ g mol⁻¹ = mol.
• Concentration unit slips
  Concentration (mol dm⁻³) = moles ÷ volume in dm³. 1000 cm³ = 1 dm³, so divide cm³ by 1000.
• Molar gas volume slips
  At s.t.p. one mole of any gas occupies 22.4 dm³. Volume = moles × 22.4 dm³. Equal volumes ↔ equal moles (Avogadro).
• Gives the percentage of the rest of the compound
  % of element = (number of its atoms × Ar) ÷ Mr × 100. Put the asked-for element on top. Check: the element with the bigger share of the mass should have the bigger percentage.
• 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.
• Takes one factor as the whole answer
  Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
• Limiting reagent errors
  Convert each reactant to moles, divide by its coefficient; the smallest value is the LIMITING reagent and determines the product amount.
• Ignores the given amount and uses one mole
  Convert the given quantity to moles first (n = m ÷ M or V ÷ 22.4), then apply the mole ratio from the equation, then convert back.
• 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.
• 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.
• Uses the wrong arithmetic operation
  Write the relationship in words before substituting (crystals = solubility at high T − solubility at low T; neutrons = A − Z; three doublings = 2³). Then check the size of the answer.
• Ignores stoichiometric ratio
  Always balance the equation first, then use the coefficients as the mole ratio between the substances asked about.
• 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.