Percent Yield Notes

General Announcements

• Exams:
  • Chemistry exam results to be posted this week.
  • Opportunity to rewrite the midterm during finals week in June.
    • Different version but same types of questions.

Course Progress

• Unit 1:
  • 50% of the chemistry course.
  • Currently finishing section five: percent yield.
• Portfolio 4:
  • Due on Thursday.
  • All questions can be completed after today's lesson.
• Online Assignments:
  • Two class periods dedicated to working on it.
  • Notes and book can be used.
  • One attempt, not timed, can go in and out of it.
• Upcoming Test:
  • On stoichiometry (section five only).
  • Includes limiting reagent, percent yield, and stoichiometry.

Schedule

• Today: Notes on percent yield.
• Tomorrow: Work on Portfolio 4.

Percent Yield

• Definition: A measure of the efficiency of a reaction in changing reactants to products.
• Real-world vs. Paper:
  • Reactions on paper don't always happen the same way in a lab.
  • Most reactions do not produce as much product as expected (less than 100% yield).

Types of Error

• Random Error (Human Error):
  • Stems from the actions of the person performing the experiments.
  • Can be minimized by careful work and practice.
  • Examples:
    • Not allowing a slow reaction to go to completion.
    • Not carrying out a reaction under the ideal temperature and pressure conditions.
    • Weighing too much or too little.
    • Spilling material.
    • Misreading a value from a measuring device.
• Parallax Error:
  • Not reading at eye level.
  • To avoid, crouch down at eye level or lift the graduated cylinder to eye level.
  • Read at the meniscus line (bottom of the curve).
• Systematic Error:
  • Arises from the quality of the materials and the equipment used.
  • Can be minimized by using better equipment or ensuring equipment is calibrated properly and clean.
  • Examples:
    • Devices that measure improperly or inaccurately.
    • Chemicals that are not pure or are contaminated.

Theoretical vs. Actual Yield

• Calculating Percent Yield: Involves theoretical yield and actual yield.
• Theoretical Yield:
  • What you calculate on paper using stoichiometry.
  • The expected amount; the maximum amount that can form under ideal conditions.
  • Assumes that there is no error, and it can be predicted through calculation on paper.
• Actual Yield:
  • What you actually get in the experiment; the measured amount.
  • Usually given as you can't predict this.
  • Affected by errors.
  • May be larger or smaller than theoretical.
    • Larger if the product is not completely dry before the final weighing.
    • Smaller if you lose material, incomplete the reaction, if it's impure starting materials.

Formula for Percent Yield

• $Percent Yield = \frac{Actual Yield}{Theoretical Yield} \times 100\%$

Example Problem

• Given the reaction: $CaCO<em>3(s) \rightarrow CaO(s) + CO</em>2(g)$
• What is the percent yield if 24.8 grams of calcium carbonate is heated and 13.1 grams of calcium oxide is recovered?

Steps to Solve:

1. Stoichiometry to find the theoretical yield of calcium oxide.

   • Given: 24.8 grams of $CaCO_3$.
   • Required: Mass of $CaO$ (theoretical yield).

2. Road Map

   • Mass of $CaCO<em>3$ $\rightarrow$ Moles of $CaCO</em>3$
   • Moles of $CaCO_3$ $\rightarrow$ Moles of $CaO$
   • Moles of $CaO$ $\rightarrow$ Mass of $CaO$

3. Step 1: Convert mass of $CaCO_3$ to moles.

   • $Moles = \frac{Mass}{Molar Mass}$
   • Molar mass of $CaCO_3$ = 100.09 g/mol.
   • Moles of $CaCO_3$ = $\frac{24.8 \text{ grams}}{100.09 \text{ g/mol}}$
   • Moles of $CaCO_3$ = 0.248 moles.

4. Step 2: Mole ratio to find moles of $CaO$.

   • $Moles<em>{\text{required}} = Moles</em>{\text{given}} \times \frac{R}{G}$
   • $Moles \ CaO = 0.248 \text{ moles } CaCO<em>3 \times \frac{1 \text{ mol }CaO}{1 \text{ mol }CaCO</em>3}$
   • Moles of $CaO$ = 0.248 moles.

5. Step 3: Convert moles of $CaO$ to mass.

   • $Mass = Moles \times Molar Mass$
   • Molar mass of $CaO$ = 56.08 g/mol.
   • $Mass \ CaO = 0.248 \text{ moles } \times 56.08 \text{ g/mol}$ = 13.9 grams.

6. Calculate Percent Yield:

   • $Percent Yield = \frac{Actual Yield}{Theoretical Yield} \times 100\%$.
   • Actual yield = 13.1 grams.
   • Theoretical yield = 13.9 grams.
   • $Percent Yield = \frac{13.1}{13.9} \times 100\% = 94.2\%$.

7. Result: The experiment was 94.2% successful.