CHEMISTRY LAB



LAB: Stoichiometry of a Reaction between Vinegar and Baking Soda

Introduction

In class you have learned how to calculate how much of a chemical product you can make when you mix measured amounts of chemical reactants. In this lab, you will calculate the theoretical yield of how much product will be made. You will then measure the actual amount made and calculate the percent yield recovered.

The reaction you will be working with should be very familiar. It is often the source of the "eruption" of a volcano in an elementary school project. You will be mixing NaHCO3 (sodium bicarbonate or baking soda) and CH3COOH (acetic acid or vinegar). The reaction will generate H2CO3 (carbonic acid, which breaks up into water and carbon dioxide gas) and NaCH3COO (sodium acetate), which is a food preservation additive.

Pre-lab Questions

Suppose we mix 25.00 g of Na2CO3 (aq) with excess CaCl2 (aq).

1. Write the balanced chemical equation for this reaction below:

2. What will the theoretical yield of CaCO3 (s) be?

3. If our actual yield from this reaction was 24.58 g of CaCO3 (s), what is the percent yield of the reaction?

percent yield = (actual yield/theoretical yield) x 100%

4. Is the answer in question 3 reasonable? If so, explain why you think it is reasonable. If not, explain what is wrong with it and discuss possible reasons you might get this answer in the laboratory.

5. What are some factors that might cause our percent yield to be greater than 100%? What are some factors that might cause it to be less than 100%? List specific examples in your answer.

Part 1: Computing amounts of reactants that we need

In this lab, you will be reacting NaHCO3 (sodium bicarbonate) with an excess of CH3COOH (acetic acid). By doing this you will ideally ensure that you will get 100% actual yield for the reaction.

For our reaction we will need to use exactly 0.03 moles of sodium bicarbonate. If we use much more than 0.05 moles of sodium bicarbonate, then you risk not having excess acetic acid.

1. Calculate how many grams of sodium bicarbonate we will need for this lab. Show your work.

2. Using the information given in the introduction, write the balanced chemical equation for the reaction between NaHCO3 (sodium bicarbonate) and CH3COOH (acetic acid) below. (Hint: write carbonic acid as the products of its decomposition, CO2 + H2O).

Part 2: The Reaction

1. Measure out the mass of sodium bicarbonate that you calculated you would need in Part 1 of this lab. Make sure that the amount you use is as close as you can make it to this calculated amount. Write the exact mas of sodium bicarbonate that you used here:

Mass of sodium bicarbonate used: _____________________ g

2. Mass a 250 mL beaker. You will need the mass of the empty beaker at the end of the lab.

Mass of the empty beaker: _____________________ g

3. Add the sodium bicarbonate to the pre-weighed 250 mL beaker.

4. Obtain 75 mL of acetic acid and SLOWLY add it to the sodium bicarbonate. You will observe the formation of bubbles. Wait until the bubbling subsides before adding more acetic acid. When all of the acetic acid has been added, stir for two minutes before moving on to step 5.

5. Find the mass of a boiling chip and then add it to the beaker. You will be boiling away the liquid portion of this solution. The boiling chip will help to prevent the contents of the beaker from boiling over.

Mass of the boiling chip: _____________________ g

6. When all of the liquid in the solution has boiled away, remove the beaker from the heat. The powder that you observe inside is one of the products of the reaction, sodium acetate. Once the beaker has had a few minutes to cool down to room temperature, measure and record its mass

Mass of the beaker, after the reaction: _____________________ g

7. Rinse out the beaker and any other glassware you used. All waste (except boiling chips) can go down the drain.

Analysis Questions

1. Copy down the balanced chemical equation of the reaction that we did in this lab from Part 1, Question #2.

2. Using the exact mass of sodium bicarbonate that you measured in Part 2, Step 1 and the balanced chemical equation that you wrote in the problem above, calculate the theoretical yield of sodium acetate in this reaction. Show your work.

3. Calculate the actual yield of sodium acetate that you recovered in this lab, using the mass of the empty beaker and the mass of the beaker after the reaction. Show your work.

4. Using the actual yield of sodium acetate that you measured in step 3 and the theoretical yield of sodium acetate that you calculated in step 2, calculate the percent yield of sodium acetate recovered in this lab.

5. Was your percent yield of sodium acetate 100%? What factors do you think caused any error that you found? Explain, using specific examples.

6. Do you think it is common for chemists to get 100% yields for chemical reactions? Why or why not?

7. If you had to do this lab again, what would you do differently to improve your answers? Explain using specific examples.

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