CHEMISTRY LAB



LAB: Stoichiometry of a Reaction between Vinegar and Baking Soda

Introduction

The reaction you will be working with in this lab 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.

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. Before doing this lab, you will calculate the theoretical yield of one of the products of this reaction. The theoretical yield is the maximum amount of product that can be formed. You will then do the lab and measure the actual yield of product made. This will allow you to calculate the percent yield of your reaction.

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Pre-lab Questions

Suppose we mix 25.00 g of Na2CO3 with excess CaCl2.

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

|grams | | | | |

|molar mass | | | | |

|moles | | | | |

2. Use the box above to determine the theoretical yield of CaCO3 in grams. Circle your answer in the box above.

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

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 list two possible reasons you might get this answer in the laboratory.

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% 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.

5. Calculate how many grams of sodium bicarbonate we will need for this lab by converting 0.03 moles of sodium bicarbonate to grams. Show your work.

Teacher Initials: ___________

6. 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. This means you will have three products for your reaction).

Part 2: The Reaction

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

Mass of sodium bicarbonate used: _____________________ g

8. 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

9. Add the sodium bicarbonate to the 250 mL beaker.

10. Measure 45 mL of acetic acid into a graduated cylinder and then SLOWLY add it to beaker with 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 11.

11. Find the mass of one 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

12. When all of the liquid in the solution has boiled away, turn off the gas. 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

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

Part 3: Analysis Questions

14. Copy down the balanced chemical equation from Part 1, Question #2 and the exact mass of sodium bicarbonate that you measured in Part 2, Question #1 into the box below.

Then calculate the theoretical yield of the mass of sodium acetate in this reaction.

|grams | | | | |

|molar mass | | | | |

|moles | | | | |

15. Calculate the actual yield of the mass of sodium acetate that you recovered in this lab. To do this, you will need to use the mass of the empty beaker, the mass of the boiling chip, and the mass of the beaker after the reaction. Show your work.

16. 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. Show your work.

17. Was your percent yield of sodium acetate 100%? List at least two factors that you think caused your percent yield to be greater or less than 100%.

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