Limiting Reactant Essay
In chemical reactions, the significance of knowing the limiting reactant is high. In order to increase the percent yield of product, increasing the limiting reactant, possibly, is the most effective. In this experiment we were able to calculate limiting reactants from the reaction of CaCl2. 2H2O + K2C2O4.H2O(aq). As a group, we obtained our salt mixture of calcium chloride and potassium oxalate, and weighed the mixture. We were able to make an aqueous solution from the mixture and distilled water. We boiled and filtered off the solution, leaving the precipitate. Once the precipitate was dried overnight, it was weighed and the mass was measured. Then we calculated the moles of the precipitate. From these calculations, we established moles of the limiting reactant, were the same amount of moles in the product based on the stoichiometrically balanced equation. Next the percent yield of the limiting reactant was calculated. In Part B of this experiment, two solutions were added to the aqueous product in order to determine the limiting reactant. Once each solution was added, we were able to visibly see the precipitate forming when 0.5 M CaCl2 was added. This made us conclude the limiting reactant was in fact CaCl2. Introduction
Stoichiometry is a section of chemistry that involves using relationships between reactants and/or products in a chemical reaction to determine desired quantitative data. Doing stoichiometry can calculate masses, moles, and percent’s with a chemical equation. The use of stoichiometry is how we were able to find the limiting reagent in this lab. We know that the limiting reagent is the chemical that will be used up first. Two factors affect the yield of product in a chemical reaction: the amounts of starting materials and the percent yield of the reaction. Under certain conditions such as temperature and pressure, can be adjusted to increase the yield of a desired product in a chemical reaction but because the chemicals react according to fixed mole ratios, only a limited amount of product can form from measured amounts of starting materials. A way for us to better understand this concept of the limiting reactant is to observe the reaction in our experiment. The reaction of calcium chloride dehydrate, CaCl2·2H2O, and potassium oxalate monohydrate, K2C2O4·H2O, in an aqueous solution.
For the reaction system in this experiment, both the calcium chloride and potassium oxalate are soluble salts, but the calcium oxalate is insoluble. The ionic equation for the reaction is Ca2+(aq)+2Cl-(aq)+2K+(aq)+C2O42-(aq)+3H2O(l)®CaC2O4·H2O(s)+2Cl-(aq)+2K+(aq)+2H2O(l) presenting only the ions that show evidence of a chemical reaction, formation of a precipitate, and by removing the spectator ions, no change of ionic form during the reaction, we have the net ionic equation for the observed reaction: is Ca2+(aq)+ C2O42-(aq)+H2O(l)®CaC2O4·H2O(s). In Part A of this experiment the solid reactant salts CaCl2·H2O forms and K2C2O4·H2O form heterogeneous mixture of unknown composition. The mass of the solid mixture is measured and then added to water-insoluble CaC2O4·H2O forms. The CaC2O4·H2O precipitate is collected by gravity filtration and dried, and its mass is measured. In Part B, the limiting reactant for the formation of solid calcium oxalate monohydrate is determined from two precipitation test of the final reactant mixture from Part A. The first test we tested the mixture for an excess of calcium ion with an oxalate reagent and the second test the mixture is tested again for an excess of oxalate ion with calcium reagents. Materials and Methods
1 250ml beaker
1 piece of filter paper
1-2 grams of salt mixture
A hot plate
A weighing scale
1. Experimenters obtained one 250 ml beaker and weighed it on the weighing scale and recorded the results
2. The 250 ml beaker was then filled with 1-2 grams of the salt mixture and weighed again
3. 100 ml of distilled water was added to the salt mixture
4. The beaker was placed on the hot plate and brought to a boil then removed
5. After cooling, the experimenters filtered the mixture using the filter paper and funnel
6. Experimenters left the filter paper to air dry overnight
7.The air dried filter paper was then placed on the weighing scale and results were recorded Results
In experiment A the results from the precipitation of CaC2O4 H2O from the salt mixture were obtained by weighing the items listed on Table 1 on a scale. Table 1.
Mass of Beaker (g)
Mass of Beaker and Salt Mixture
Mass of Salt Mixture (g)
Mass of Filter Paper (g)
Mass of Filter Paper and CaC2O4 H2O (g)
Mass of Air-Dried CaC2O4 H2O (g)
In Experiment B the limiting reactant was determined to be CaCl2 when two drops of the test reagent 0.5 M CaCl2 was added to the supernatant liquid in test tube 1, and a precipitate formed. Since there was a reaction, there was C2O42- in excess and Ca2+ is the limiting reactant in the original salt mixture present in test tube 1 . This was further confirmed when two drops of the test reagent .05M K2C2O4 was added to the supernatant liquid in test tube 2. There was no precipitate because Ca2+ was not present since it was the limiting reactant and instead C2O42- was in excess. Table 2.
Moles of CaC2O4 H2O precipitated (mol)
Moles of limiting reactant in salt mixture (g)
CaCl2 .0004 (mol)
Mass of limiting reactant in salt mixture (g)
CaCl2 .4995 (grams)
Mass of excess reactant in salt mixture (g)
Ca2C2O4 1.113 (grams)
Percent limiting reactant in salt mixture (%) CaCl
Percent excess reactant in salt mixture (%) K2C2O4
The data of the mass of the salt mixture was a big key for finding the moles of CaC2O4 precipitated. The molar mass of CaC2O4 H2O was 146.097 grams. The mass of the air-dried CaC2O4 H2O CaCl2, was .664g as recorded in table 1. Using a calculation of .664 x 1 mole / 146.097 a result of .0045 mol was recorded in table 2. The test done in Experiment B allowed us to know without any calculations that Ca2+ is the limiting reactant. This allowed us to conclude that the moles of the limiting reactant were .0004 (mol) of CaCl2. In order to achieve the grams of the limiting reactant, the moles of the limiting reactant must be multiplied by the molar mass of the limiting reactant.
Therefore the mass of the limiting reactant was .0045 moles and multiplied by its molar mass of 111g to result in .4995g of the limiting reactant in the salt mixture. Next the mass of the excess reactant in the salt mixture was calculated using the same method as the limiting reactant except the molar mass of the excess reactant was used to result in 1.113 (grams) Ca2C2O4 . The final step in the process was to find the percent by mass of the limiting reactant. Since Experiment B allowed us to determine that Ca2+ is the limiting reactant, therefore to find the percentage composition it is necessary to divide the limiting reactant mass by the mass of the original sample then multiply by 100. This provided a result of 34%, and to find the excess percentage, this value was subtracted from 100 to yield 66% of K2C2O4 as the percent of excess reactant in salt mixture. Error Analysis
Possible errors might be attributed to careless errors in reading the scale to measure the mass of the beaker, salt mixture or filter paper. Even when proper care is taken in reading the instruments, systematic errors can present themselves in the instrument used to measure mass. Here, a calibrated scale was used to measure mass, and the systematic error is unknown since it is one of the hardest errors to detect. These two sources of errors might help explain the .1% missing from the CaCl2 and K2C2O4 salt mixture recorded in Table 2.
Precision and Accuracy
While accuracy deals with how close a measured value is to a true or accepted one, precision deals with how reproducible a given measurement is. Here the mass of the beaker, salt mixture, and filter paper are all precise because they are easily reproducible since it simply involves putting the items on a scale. If the process was repeated 50 times the results would not vary or at the least by .0001 grams based on some outside factor. The mass of the air-dried CaC2O4 H2O is accurate because it was calculated as true by subtracting the mass of the filter paper from the mass of the filter paper and the CaC2O4 H2O. Conclusion
As we have stated previously, CaCl2 was our limiting reactant based on the precipitates observed. We were able to rule out Ca2C2O4 because of the lack change in our precipitate..
It was important to note that a limiting reactant in a chemical reaction limits the amount of product that can be formed. The reaction will stop when all of the limiting reactant is consumed. The excess is the reactant in a chemical reaction that remains but there is nothing with which it can react. Taking this knowledge we have gained in appropriately observing the results, we can apply it to future experiments in chemistry in order to evaluate how much product one might want to produce in a given chemical reaction.
Reviewing other experiments, from other schools, it is apparent that the need for appropriate data collection in this type of experiment, will help in identifying the excess and limiting reagents. As was the case in UCCS’s Chem 103 Lab Manual, following the procedures and doing them in the proper order are vital to ensuring success in proper reactions.
Tro, Nivaldo. Chemistry A Molecular Approach. 3rd ed. Boston, MA: Pearson Education, Inc.;
Beran, J. A. Laboratory Manual for Principles of General Chemistry. 8th ed. Hoboken, NJ: John Wiley & Sons, Inc.; 2009 Beran, J. A. Laboratory Manual for Principles of General Chemistry. 9th ed. Hoboken, NJ: John Wiley; 2010 UC Davis ChemWiki. Stoichiometry and Balancing Reactions. http://chemwiki.ucdavis.edu/Analytical_Chemistry/Chemical_Reactions/Stoichiometry_and_Balancing_Reactions UCCS Chem 103 Laboratory Manual. Experiment 3 Limiting Reactants. http://www.uccs.edu/Documents/chemistry/nsf/103%20Expt3V-LR.pdf Masterson, W, Hurley, C. Chemistry: Principles and Reactions. 6th ed. Belmont, CA: Brooks/Cole Cengage Learning; 2009.