Topic 14: advanced stoichiometry with limited and excess reagents

We will be revisiting the chemical equation and learning how to use it to find out how much product we can make and/or reactant we need

a chemical equation works exactly the same was as a recipe: it tells us the ratio of things we need

when the actual amounts are equivalent to the ratio from the chemical equation, that means there is no limiting reagent

when one of the reactants limits the amount of product, that reactant is called the limiting reagent. the other reactant is called the excess reagent

  • the limiting reagent is completely consumed in the reaction

  • the excess reagent is not completely consumed in the reaction

How to do basic limiting reagent problems

  1. construct the equation

  2. balance the equation

  3. convert given masses (grams) of reactants to moles using their molar mass

  4. identify limiting reactant by diving the given quantity of moles by its stoichiometric coefficient in the balanced equation. tip:

    1. lowest value = limiting reactant

    2. higher value = excess reactant

  5. us the moles of the limiting reactant to calculate the theoretical yield (in moles) of the products using the stoichiometric mole ratio

  6. determine the amount of moles that the excess reactant used in the reaction

    1. determine the moles of amount of excess reactant there is left

  7. convert moles to grams if needed


but when we are doing labs what we can measure are grams not moles, so we need to know how to use stoichiometry to do gram to gram calculations about excess and limiting reagents

given: grams of substance a and B

  1. convert to moles of substance a and B using molar mass

  2. use the coefficicients of A and B from the balanced chemical equation to find out which is the limiting reactant

    1. divide moles actually present by moles in balanced equation

      1. lowest value = limiting reactant

      2. highest value = excess reactant

  3. calculate the mols of product produced given the limiting reactant

  4. convert mols to grams


steps given in video

  1. write and balance the chemical equation

  2. convert all masses from grams to mols

  3. se the stoichiometric coefficients to find the limiting reagent and predict the moles of the desired species (other reactant/s or product)

  4. convert the moles of product/other reactant (based on the limiting reagent) to grams of the product/other reactant


Percent yield calculations

theoretical yield: 100% yield. this is the amount that we predict being able to make from the stoichiometry

actual yield: what we actually produce

we report the efficiency of the reaction (how close we got to the theoretical yield) as the percent yield

NOTE: typically, when we report an amount produced, it is the actual yield