DETERMINING THE YIELD OF A CHEMICAL REACTION 

Unit 6: Reactions and Stoichiometry

Objectives:  There are two purposes for this lab:

1.      that you would determine the percent yield for a chemical reaction.

2.      that you would filter, rinse and dry a solid product from a chemical reaction in order to determine its mass.  (This procedure is called, “gravimetric analysis.”)

Introduction: In this lab you will make a precipitate through a simple double replacement reaction.  Based upon the quantities that you react together, you will predict the mass of the precipitate, CaCO₃ that will form.  You will then carry out the reaction and see how much product you actually succeed in producing, purifying and isolating in the end.  In order to determine the percent yield you will compare your theoretical mass of precipitate expected to the actual mass that you produce.  You will then consider where and in what steps an error in procedure or in handling may have caused a discrepancy in your percent from 100%.  The chemical reaction is:

CaCl₂ (aq)  + Na₂CO₃ (aq)   →  2NaCl (aq)   +  CaCO₃ (s)

Equipment and Materials:

• Calcium chloride dihydrate;  (Hint: a hydrate means that there is water in the crystal.) 

• Sodium (or potassium) carbonate (Note which you use and whether it is hydrated.)

• Distilled water

• 125 ml Erlenmeyer flask

• 2^nd Erlenmeyer flask large enough to accommodate the funnel stem

• 50 ml beaker

• filter paper

• funnel

Procedure:

1.   Accurately weigh approximately 2 grams of calcium chloride dihydrate into a 50 mL beaker and add approximately 20 mL of distilled water to dissolve the crystals.

2.   Into a 125 mL Erlenmeyer flask add approximately 4 grams of sodium carbonate and dissolve it by the addition of 50 mL of distilled water and vigorous swirling. 

3.   Add the calcium chloride solution to the Erlenmeyer flask containing the sodium carbonate.  Be sure to wash any residual solution from the beaker using distilled water. 

4.   Set the flask aside to prepare for filtration.

5.  Weigh a piece of filter paper; record this weight.  Fold the filter paper into quarters.  Separate the layers (folds) of paper to produce a pocket and put the filter paper into a funnel perched atop a second Erlenmeyer flask. 

6.  Carefully pour the calcium carbonate slurry (a suspension of a solid in a liquid) into the filter paper pocket.  Be sure to transfer all the precipitate with additional rinsing as necessary.  Allow the water to completely drain out of the filter.

7.  When the filter is (nearly) empty carefully wash the sides with two five mL portions of distilled water in order to rinse the paper of the other dissolved product ions.

8.  When the filter paper has fully drained, carefully transfer it to a beaker and place the beaker in the drying oven.  Open the filter paper slightly to aid in drying and allow the damp filter paper to dry overnight.

9.   Remove the dry filter paper from the beaker and accurately weigh and record the weight.

Data and Observations:  Record your data and observations in the course of the procedure in your Student Laboratory Notebook.  Remember that you want to note where possible experimental errors could occur. 

Analysis Questions: Answer the following questions after your data and observations on your Student Laboratory Notebook pages.  Reminder: All calculations to answer the following questions should be clearly labeled and laid out in a manner the progression and logic of which is easy to follow.

1.      Which reactant is the limiting reactant and which reactant is present in excess?

2.      What is the theoretical yield of calcium carbonate in grams?

3.      What is the percent yield for your experiment?

4.      State three possible experimental errors (not calculation errors) that would cause a group’s results to be less than 100%.

5.      State three possible experimental errors that would cause a group’s result to be greater than 100%.

Your write-up will consist of turning in the pages from you Student Laboratory Notebook.