Exploring Limiting Reactants and Percent Yield in the Laboratory

Purpose: The purpose of this lab is to find the limiting reagent/excess reagent and the percent yield of carbon dioxide in an Alka-Seltzer tablet.

Background: A limiting reagent is the reactant in a reaction that will be completely used by the end of the experiment. It will literally limit the amount of product that can be produced. The excess is the reactant that will not be able to be used completely because there’s too much of it and not enough limiting for it to react with. A simple way to understand this concept would be by comparing it to an everyday example. For instance, if you have 10 bicycle frames and 30 wheels, your limiting reactant would be the frames. Considering each bicycle will only need 2 wheels, 10 bicycles will only require a total of 20 wheels. Therefore, there will still be 10 wheels remaining after you use all the frames, making it the excess because they have no frames to attach to. This ultimately means you will only be able to build 10 bicycles compared to the 15 that could’ve been made with enough frames to fit the number of wheels. Percent yield helps show us how much of a product will be produced after a reaction is completed. It can be solved for by dividing the actual yield, which you get from actually doing the experiment, by the theoretical yield, which can be found by mathematically solving for what you should have gotten assuming you did everything perfectly, and then multiplying that by 100.

Procedure:

1. Record the mass of the sodium bicarbonate and of the citric acid in one tablet based on what it says on the back of the Alka-Seltzer packet.
2. Obtain a small beaker and fill it about halfway with water.
3. Record the mass of the beaker with water.
4. Record the mass of one Alka-Seltzer tablet.
5. Drop the Alka-Seltzer into the beaker with water.
6. Watch as it reacts and record any observations you have.
7. Wait until the tablet fully dissolves.
8. For 10 minutes, let the beaker with the water and tablet sit there.
9. Record the mass of the beaker with the water and Alka-Seltzer.

Materials:

• one Alka-Seltzer tablet
• distilled water
• scale
• beaker.

Observations while dissolving

• intense fizzing and bubbling
• water droplets splashing on the sides due to the fizzing
• white residue on the perimeter of the upper walls of the beaker
• bubbles rising after the tablet fully dissolved Mass after tablet fully dissolved 100.77 g.

Limiting reactant

The limiting reagent in this experiment is the because it produced less carbon dioxide, implying that it was the reactant that limited the reaction.

Excess reactant

The excess reagent is the because it produced a greater amount of carbon dioxide, meaning there would be some of it left over once the reaction stops.

Amount of limiting used

The amount of citric acid used was 1000 milligrams or 1 gram because all of it was used in the reaction. Amount of excess used 1000 mg X X X X = 1.312 g = 1 g

Amount of limiting left over

Zero grams of citric acid was left over because it was all used. Amount of excess left over 1916 mg X = 1.916 g 1.916 g (total) – 1 g (used) = 0.916 g = 1 g Percent yield of % yield = X 100% = X 100 = 70%

Conclusion: My lab group was able to accomplish the purpose of this lab. One error we could have experienced would be not allowing the tablet to fully dissolve. Although there are signals that help notify us when the substance has dissolved, it is possible that we began timing the 10 minutes too early because we could not see the dissolving on a chemical level, which ultimately wouldn’t have given the carbon dioxide enough time to be emitted from the beaker completely. Furthermore, as the Alka-Seltzer tablet was dissolving, it was fizzing vigorously, which caused liquid to jump out of the beaker. Because of this, it is possible that small amounts of mass were lost, which would have reduced our entire mass. One way to improve this lab would be to do it with other substances to test how they react with each other and what reactants would be the limiting/excess ones. For example, doing an experiment combining aluminum and copper (II) chloride to find the percent yield of copper would be a good way to gain a better understanding of how limiting/excess reactants work. It would be good practice for actually solving for the yield, rather than being given the actual yield and simply doing math to find the experimental.