Mass of a Reaction Product

Sodium carbonate reacts with hydrochloric acid producing bubbles

OBJECTIVE: To use the principles of stoichiometry to determine the theoretical yield of a simple reaction, measure the actual yield, and calculate the per cent yield.

This laboratory is part of a entire unit on teaching stoichiometry. You can access the complete lesson plans with information on their use, and links to other worksheets, labs, and activities at http://www.jce.divched.org/article/stoichiometry-easy

Concepts: 

Stoichiometry, Percent yield, Theoretical Yield, Actual Yield

Time required: 

One 50-minute period to perform the lab. One additional period to perform the calculations (optional). Often more able students will have time to begin some calculations at the end of the lab experiment.

Materials: 

APPARATUS
Centigram balance
2 Thin Beral pipets
1 - Condiment cup
Plastic wrap
Goggles
1 - 100 mL Beaker
2 - 50 mL or 100 mL Beakers

REAGENTS
Solid sodium carbonate (Na2CO3)
3M Hydrochloric acid (HCl)

Background: 

BACKGROUND: A balanced chemical equation includes a great deal of useful information. Not only does it tell you, in a concise manner what the reactants and products are, but it also tells the relative amounts of each substance.

The coefficients in the balanced chemical equation give the mathematical relationships that exist among the moles of each substance involved. When the coefficients are expressed as ratios you can predict the amount of a product that will form from a given amount of a reactant (as well as the amount of one reactant to use given the amount of another, and the amounts of reactants needed to create a certain amount of a product). Using the coefficients of a balanced equation in this way is the essence of stoichiometry.

In this experiment you will react a measured amount of sodium carbonate with an excess of hydrochloric acid (that is, more than enough to use up). The products of the reaction are sodium chloride, water, and carbon dioxide (a gas). The carbon dioxide gas will bubble out of the solution and be lost. Measuring the decrease in mass gives the amount of carbon dioxide produced. Using the principles of stoichiometry you can then calculate the mass of carbon dioxide that should have been formed, and determine the percent yield of the experiment. Of course, if everything works exactly perfectly, the percent yield should be 100% – that is, the mass that is predicted to be formed should be the same as what is actually made.

Procedure: 

1. Put on your goggles. Take care in using the 3M hydrochloric acid as it is corrosive. If you spill it on the lab table you can neutralize it promptly with dilute sodium bicarbonate solution. If you spill some on your hand rinse thoroughly and promptly with water in the sink.

2. Use the centigram balance to measure the mass of the condiment cup. If you are using a digital balance, use the “tare” button to make certain the balance reads “0.00 g” at the start, then place the cup on the center of the balance pan. Record the mass of the empty cup on the report sheet.

3. If you are using a triple beam balance, set the sliders to add 0.50 g to the current mass. If you are using a digital scale, press “tare” again to reset the display to 0.00 g. Add something close to 0.50 g (between 0.45 g and 0.55 g) of sodium carbonate to the condiment cup. Record the exact mass of the cup with sodium carbonate (if you are using a triple beam balance), or of the added sodium carbonate (if you are using a digital scale). Be careful not to spill any sodium carbonate on the balance pan.

4. Wrap a small piece (a 4 or 5 inch square) of plastic wrap over the top of the cup.

5. Using a very sharp pencil or the tip of a scissors, poke three small holes in the plastic wrap that are just big enough to admit the tip of a Beral pipet. Distribute the holes so that they are equally spaced around the plastic on top of the cup about ¼ inch from the edge and about  ¼ inch from each other.

6. Go to the reagent table and get two pipets in one of the small beakers (50 mL or 100 mL) that you find there. Fill both pipets completely with 3M HCl solution and place them bulb down (tip up) in the beaker. Use special care when handing and transporting the hydrochloric acid.

7. Reset the balance to zero (0.00 g) and find the mass of all the equipment you have used so far: The beaker with the two filled pipets and the condiment cup with sodium carbonate in it and the plastic wrap over it. Record this mass.

8. Begin the reaction: Carefully insert the top of one of the Beral pipets through one of the holes of the plastic wrap and lower it about halfway into the cup of sodium carbonate. Add the acid one drop at a time to the sodium carbonate. Add the acid slowly and wait for the bubbling to slow down before adding more acid. After adding about 10 drops of acid, gently swirl the solution to mix it. Continue to add acid and swirl gently until the reaction stops. You may have to use acid from the second pipet. Make certain that there are no tiny pieces of unreacted solid in the condiment cup.

9. Record your observations on the Report Sheet.

10. Add two more drops of acid to make certain that all of the sodium carbonate has reacted.

11. Reset the balance to zero (0.00 g) and again find the mass of all the pieces of equipment you have used (listed in step 7).

12. Place both pipets with any remaining acid back into their beaker and return them to the reagent table where you found them.

13. Slowly spill the reaction solution down the drain. Rinse it down with water. Thoroughly rinse out the condiment cup, dry it, and return it to its place.

14. Wash your hands thoroughly. Begin the calculations as instructed at your desk.

Questions: 

1. What is the actual yield of carbon dioxide produced?

2. The word equation for the reaction is: solid sodium carbonate reacts with hydrochloric acid solution to produce water, sodium chloride in solution, and carbon dioxide gas. On the line below, write and balance the chemical equation using chemical symbols.

3. Use the periodic table to find and record the molar mass of each of the following two reagents to at least four significant figures:

    a) sodium carbonate: 1 mol = _______ g     b) carbon dioxide: 1 mole = _______ g

4. Use the steps of stoichiometry to determine how many grams of carbon dioxide you would have produced (if everything in this experiment worked perfectly) given the mass of sodium carbonate that you added to the condiment cup. Show your work on the next page and give your answer to the correct number of significant figures.

5. What is the theoretical yield of carbon dioxide?   _______________ g

6. Determine the percent yield of carbon dioxide. Show your work and use a reasonable number of significant figures.

7. Name and state the scientific law that requires your percent yield to be 100% if the entire reaction was perfect, and there were no errors in your procedure or measures.

8. Your teacher will help you to calculate an approximate percent uncertainty for the entire experiment. Based on your calculated percent yield and the estimated acceptable uncertainty given by your teacher, is the law you stated in question seven valid for this experiment? Why or why not?

Preparation: 

Sodium carbonate is best available in a vial or small jar at each lab table. The hydrochloric acid can be in beakers at two or three locations in the classroom for students to fill their pipets. This is very quick and easy, so it is not necessary to have HCl at each table.

Cut plastic wrap into squares that are about 4 x 4 or 5 x 5 inches. This is most easily done if you cut several pieces of plastic wrap to about 11 x 8.5 inches and then place a sheet of scratch paper between the sheests with one sheet of paper on top and one on the bottom. Then cut the plastic wrap "sandwich" into quarters. In this way the plastic wrap can be easily distributed and will not stick to itself.

An alternative to the plastic wrap is to use lids that fit the condiment cups. Use a small awl, or the point of a protractor to poke three holes in the lids.

 

Attribution: 

Developed in conjunction with Courtney Lantz, Pacifica High School (retired).