Moles

Traps in Moles

• 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.
• 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.
• 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.
• 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.
• 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.
• Inverts n = m/M
  Moles = mass ÷ molar mass. Check units: g ÷ g mol⁻¹ = mol.
• 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.