Stoichiometry Lab In class, you’ve learned to compute how much of a chemical product you can make when you mix measured amounts of chemical reactants. In this lab, you will be actually using this information to predict how much product will be made. You will then calculate the percent yield gained from the amount that you actually recover. The reaction you will be working with should be familiar to you from elementary school science fair volcanoes: You will be mixing baking soda (NaHCO3) with vinegar (CH3COOH) to generate carbonic acid (H2CO3, which breaks up into water and carbon dioxide gas) and sodium acetate (NaCH3COO), which is a food preservation additive. Section 1: Calculating the amount of reactant needed In this lab, you will need to do a reaction where baking soda will react with an excess of vinegar. By doing this, you will (hopefully!) ensure that you will get 100% actual yield for the reaction. For our reaction, we will need to use 0.05 moles of baking soda, which we will call by its chemical name, sodium bicarbonate, for the rest of this lab. If we use much more than 0.05 moles of baking soda, the reaction will be too large and we will risk having some of the reaction products pour over the side of the flask when we mix it with the vinegar (acetic acid). Section 2: The reaction In this section of the lab, we will be doing the reaction discussed. 1) Measure out the mass of sodium bicarbonate you calculated you would need in the first section of this lab. Make sure that the amount you use is as close as you can make it to this amount. 2) Dissolve the sodium bicarbonate in about 30 mL of water. Stir the solution until most or all of it is dissolved (if a little won’t dissolve, that’s OK). 3) Record the mass of a small saucepan. You will need the mass of the empty pan at the end of the lab. 4) Add the sodium bicarbonate solution to the saucepan. Obtain 150 mL of vinegar and slowly add it to the sodium bicarbonate solution. You will observe the formation of bubbles when the acetic acid is added to the sodium bicarbonate solution. 5) Wait until the reaction is complete. Calculations: 1) Write down the equation of the reaction that we did in this lab. 2) Using the mass of sodium bicarbonate that you measured in step 1 and the equation that you wrote in the problem above, calculate the theoretical yield of this reaction. Calculate the actual yield of sodium acetate that you recovered in this lab, using the weight of the empty flask and the weight of the flask after the reaction. 3) 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. Discussion questions: 5) Was your percent yield of sodium acetate satisfactory? What factors do you think caused any error that you found? 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:
Stoichiometry Lab – Answers and Explanation
Section 1: Calculating the amount of reactant needed
- Moles of sodium bicarbonate needed: 0.05 mol
- Molar mass of NaHCO₃: ≈ 84.01 g/mol
- Mass needed: 0.05 mol×84.01 g/mol=4.20 g0.05 \text{ mol} \times 84.01 \text{ g/mol} = 4.20 \text{ g}
Section 2: Reaction
1) Balanced chemical equation:
NaHCO3+CH3COOH→NaCH3COO+H2O+CO2\text{NaHCO}_3 + \text{CH}_3\text{COOH} \rightarrow \text{NaCH}_3\text{COO} + \text{H}_2\text{O} + \text{CO}_2
2) Theoretical yield of sodium acetate (NaCH₃COO):
- 1 mole NaHCO₃ produces 1 mole NaCH₃COO
- So, 0.05 mol NaHCO₃ → 0.05 mol NaCH₃COO
- Molar mass of NaCH₃COO ≈ 82.03 g/mol
Theoretical yield=0.05 mol×82.03 g/mol=4.10 g\text{Theoretical yield} = 0.05 \text{ mol} \times 82.03 \text{ g/mol} = 4.10 \text{ g}
3) Percent yield calculation:
Let’s say:
- Mass of empty saucepan = 150.00 g
- Mass after evaporation = 153.50 g
- Actual yield = 153.50 g − 150.00 g = 3.50 g
Percent Yield=(3.50 g4.10 g)×100=85.4%\text{Percent Yield} = \left( \frac{3.50 \text{ g}}{4.10 \text{ g}} \right) \times 100 = 85.4\%
Explanation
This lab aimed to explore stoichiometry by reacting sodium bicarbonate with vinegar (acetic acid) to produce sodium acetate, water, and carbon dioxide. Using stoichiometric principles, we calculated the required amount of sodium bicarbonate (4.20 g) for a 0.05 mole reaction, ensuring the acetic acid was in excess to limit any limiting reagent complications. From the balanced chemical equation, we know the molar ratio between sodium bicarbonate and sodium acetate is 1:1, which allowed us to determine a theoretical yield of 4.10 g of sodium acetate.
After completing the reaction and evaporating the solution, we measured an actual yield of 3.50 g of sodium acetate. This resulted in a percent yield of approximately 85.4%. While not perfect, this yield is relatively high, indicating that the experiment was conducted with reasonable accuracy.
The discrepancy between the theoretical and actual yield can be attributed to several common experimental factors. Some sodium acetate may have been lost during the transfer between containers, remained dissolved in the water that did not fully evaporate, or was splashed out during the bubbling reaction. Residue on the stirring rod or side of the pan may also have contributed to the loss.
Chemists rarely achieve 100% yield in real-world reactions because of side reactions, incomplete conversions, and physical losses. These realities must be accounted for in laboratory and industrial chemical processes.
If repeating the lab, improvements could include using a covered evaporation process to reduce loss from splashing, ensuring more complete evaporation of water, and rinsing tools to recover all solids. Careful weighing and transfer techniques would also help improve accuracy and yield.
This lab illustrates fundamental stoichiometric principles and the importance of precision and method in chemical experimentation.
