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14.4: Percent Yield

Difficulty Level: At Grade Created by: CK-12

Student Behavioral Objectives

The student will:

• define theoretical and actual yield.
• explain the difference between theoretical and actual yield.
• calculate percent yield (reaction efficiency).

Timing, Standards, Activities

Timing and California Standards
Lesson Number of 60 min periods CA Standards
Percent Yield 1.5 3f

Activities for Lesson 4

Laboratory Activities

1. None

Demonstrations

1. None

Worksheets

1. Percent Yield Worksheet

1. None

Answers for Percent Yield (L4) Review Questions

• Sample answers to these questions are available upon request. Please send an email to teachers-requests@ck12.org to request sample answers.

Multimedia Resources for Chapter 14

This website provides a stoichiometry game.

This website contains various resources, including PowerPoint lectures, on many topics in chemistry, including one on stoichiometry.

This website has lessons, worksheets, and quizzes on various high school chemistry topics. Lesson 9-3 is on solving mass-mass problems.

This website has several video lessons on performing stoichiometry calculations, including problems with a limiting reactant.

This video is an electronic blackboard presentation of the calculation of a percent yield.

Teacher's Pages for Mass-Mass Relationships in a Chemical Change

Lab Notes

The setup for this lab is easy. Prepare the \begin{align*}6 \ M\end{align*} \begin{align*}HCl\end{align*} by diluting the stock \begin{align*}12 \ M\end{align*} acid with a \begin{align*}\frac{50}{50}\end{align*} (v:v) volume of water. Prepare enough so that each lab team has access to about \begin{align*}10 \ mL\end{align*}.

The most common error with this lab is to add too little or too much acid. The students will often leave unreacted \begin{align*}NaHCO_3\end{align*} on the bottom of the evaporating dish because it is difficult to see the difference in color between the dish and the \begin{align*}NaHCO_3\end{align*}. Have students swirl the acid/\begin{align*}NaHCO_3\end{align*} mixture a little and look closely, the powder will appear as a sediment.

The \begin{align*}6 \ M\end{align*} acid often has a way of reminding us that chemical contamination can be dangerous. If a student reports itching, wash the affected area with copious amounts of water and send the student to the nurse if warranted.

Lab – Mass-Mass Relationships in a Chemical Change

Background Information

When chemical reactions occur, the amount of product created can be theoretically predicted if you know the amounts of reactant used. This phenomenon is based on the law of mass-energy conservation. Since you cannot create or destroy matter in a chemical change, the sum of the masses of reactants and the sum of the masses of products must be equal. For example:

\begin{align*}C+O_2 & \rightarrow CO_2\\ 12g \ 32g & = 44g\end{align*}

Since this equation is balanced, the coefficients in front of each reactant are 1. One mole of \begin{align*}C\end{align*} must react with 1 mole of \begin{align*}O_2\end{align*} to produce 1 mole of \begin{align*}CO_2\end{align*}. The object of this experiment is to prove this in the laboratory. The reaction you will be performing is as follows:

\begin{align*}NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\end{align*}

As you can see, the coefficients in this reaction are also ones. One mole of \begin{align*}NaHCO_3\end{align*} reacted with one mole of \begin{align*}HCl\end{align*} will produce one mole of \begin{align*}NaCl\end{align*}, one mole of \begin{align*}H_2O\end{align*}, and one mole of \begin{align*}CO_2\end{align*}. You will see that the number of moles of \begin{align*}NaHCO_3\end{align*} will be equal to the number of moles of \begin{align*}NaCl\end{align*} produced. The \begin{align*}CO_2\end{align*} is removed by letting it escape into atmosphere, and the \begin{align*}H_2O\end{align*} is removed by evaporating it in a drying oven. The mass of salt can then be determined.

Purpose

The purpose of this activity is to study the relationships between the numbers of moles of reactant used, number of moles of products obtained, and coefficients used to balance a chemical reaction.

Apparatus and Materials

• Evaporating dish
• \begin{align*}10 \ mL\end{align*} graduate
• Watch glass
• Disposable pipette
• Electronic balance
• Goggles and apron
• Electric Oven
• Scoopula

Safety Issues

\begin{align*}6 \ M\end{align*} HCl is VERY strong acid! It will cause a nasty chemical burn if you get it on your skin, and will put holes in your clothing. Wear goggles and apron, and if you get it on your skin, you will begin to itch immediately. Wash it off with plenty of water.

Procedure

1. Weigh a clean, dry evaporating dish + watch glass on the electronic balance. Record the mass.
2. Weigh out \begin{align*}2.0 \ g \pm 0.10 \ g\end{align*} of \begin{align*}NaHCO_3\end{align*} into the evaporating dish, and record the mass.
3. Measure \begin{align*}5.0 \ mL\end{align*} of \begin{align*}6 \ M\end{align*} \begin{align*}HCl\end{align*} into a \begin{align*}10 \ mL\end{align*} graduate.
4. Place the watch glass on top of the evaporating dish, curve side down. Then add the \begin{align*}HCl\end{align*} through the remaining opening by transferring it with a disposable pipette. Add all of it. Swirl the dish slowly. Record what you see. The reaction is complete when no solids remain. Add more acid drop wise if the reaction is not complete.
5. Place the dish, the liquid, and the watch glass into an \begin{align*}110^\circ C\end{align*} oven for 24 hours.
6. Record the mass of the dish, the watch glass, and the remaining residue. Record this mass.

Data

Mass of empty dish + watch glass ___________________g

Mass of \begin{align*}NaHCO_3\end{align*} ___________________g

Mass of dish + watch glass + \begin{align*}NaCl\end{align*} ___________________g

Moles of \begin{align*}NaHCO_3\end{align*} ___________________g

Moles of \begin{align*}NaCl\end{align*} ___________________g

Post-Lab Questions

1. From your balanced equation, what is the mole ratio between the \begin{align*}NaHCO_3\end{align*} and \begin{align*}NaCl\end{align*}? How does it compare with the mole ratio from your experiment?
2. Suppose you had started with \begin{align*}20 \ moles\end{align*} of \begin{align*}NaHCO_3\end{align*}, how many moles of \begin{align*}NaCl\end{align*} would you expect to be formed? Explain.
3. Do the results of your experiment support the law of conservation of mass-energy? Explain.

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