Amount of substance common mistakes

Common mistakes

• Skipping equation balancing

  Using an unbalanced equation for stoichiometric calculations.

  Fix itAlways balance the chemical equation before performing any mole or mass calculations to ensure atom conservation.

• Using mass directly instead of moles to find the limiting reagent

  Comparing the masses of reactants without converting to moles, leading to incorrect identification of the limiting reagent.

  Fix itAlways convert masses to moles using the molar mass before applying the stoichiometric ratios from the balanced equation.

• Using the wrong theoretical yield

  Students sometimes calculate the theoretical yield using the mass of product instead of the mass of the limiting reactant.

  Fix itAlways start from the balanced equation, identify the limiting reagent, and use its stoichiometric coefficient to compute the theoretical yield.

• Incorrect calculation of atom economy

  Failing to use the correct mass of desired product or total mass of reactants.

  Fix itAlways ensure to correctly identify the masses involved in the calculation.

• Confusing Percentage Yield with Atom Economy

  Students often confuse percentage yield with atom economy, thinking they measure the same thing.

  Fix itRemember that percentage yield focuses on the actual versus theoretical yield, while atom economy assesses the efficiency of reactants being converted into useful products.

• Incorrect use of stoichiometry

  Assuming a 1:1 ratio when the balanced equation shows a different ratio.

  Fix itAlways balance the equation and use the correct mole ratio when converting between volumes and concentrations.

• Incorrect Mean Titre Calculation

  Students often forget to average all titre values correctly.

  Fix itEnsure to add all titre values and divide by the total number of titres to find the mean.

• Misinterpreting the indicator’s colour change

  Students often choose an indicator whose transition range does not overlap the equivalence point, leading to an incorrect endpoint.

  Fix itAlways match the indicator’s pH transition range to the expected pH at the equivalence point of the reaction.

• Misinterpreting stoichiometric coefficients

  Assuming a 1:1 ratio for all acid–base reactions regardless of the balanced equation.

  Fix itAlways refer to the balanced equation to determine the correct stoichiometric ratio before performing calculations.

• Misinterpreting the volume of titrant as the volume of analyte

  Students often write the volume of titrant as the volume of the solution being analysed, leading to incorrect concentration calculations.

  Fix itAlways distinguish between the volume of the analyte (the solution being titrated) and the volume of the titrant (the standard solution added). Use the correct volume in the molarity equation: CₐVₐ = C_bV_b.