Section
Introduction
The purpose of this lab was to investigate the stoichiometry and find out the mole ratios of simple acid-base reactions experimentally. The reactions studied in this experiment were the following:
HCl + NaOH → H2O + HCl
H2SO4 + 2NaOH → 2H2O + Na2SO4
H2SO4 + Ba(OH)2 → 2H2O + BaSO4
HCl + Ba(OH)2 → 2H2O + BaCl2
In the chemical reactions, the compounds react in distinct molar proportions or ratios which are constant to the peculiar reaction. The ratios determining the quantities of substances reacting are called stoichiometric coefficients. The reactions between strong acids and bases are relatively simple and non-reversible reactions the “end” of which can be easily judged.
The objectives of the lab were attained using the method of quantitative titration. It consists in adding a solution of one compound by drops to the solution of another one containing an indicator. When the amount of reactant added reaches the amount required by the stoichiometry of the reaction, the further addition will cause the permanent change of color (due to indicator). Quantitative titration is widely used in industry in quality control chemical labs. It is used to analyze blood and urine by pharmacists, to determine water, fat, and vitamin content of food in the food industry, and in a variety of other fields of industry (Bandos, 2016).
Procedure
Firstly, the buret was assembled and prepared for the experiment. It was rinsed with the stock NaOH solution and filled with it. Into the 50 mL Erlenmeyer flask, 10 mL of HCl solution was added with a volumetric pipette. The solution was diluted with deionized water (approximately 15 mL), and 2 drops of phenolphthalein indicator were also added. The NaOH solution was added dropwise from the buret to HCl solution until the color of the solution permanently changed to pink and the final volume on the buret was recorded. Fresh portion of the analyte solution was prepared in the flask, and the titration was repeated. The same two-trial procedure was repeated using H2SO4 as an analyte (also 10.00 mL). For the last two titrations, NaOH solution in the buret was replaced with Ba(OH)2 solution. The same two-trial titrations with NaOH as the titrant and HCl and H2SO4 as analytes were performed using Ba(OH)2 as the titrant. The solutions which are used in the lab are mild irritants, so the safety glasses were worn during the lab. If the traces of the solutions get on the clothes or skin, they should be rinsed off with tap water.
Data and Results
The stock concentrations of the solutions used in this lab are presented in Tab. 1:
The experimental data, as well as the results of the calculations, are shown in Tab 2.
Calculations
The sample calculations will be shown for the NaOH + HCl titration.
Equation 1. nHCl=CHCl×VHCl=0.0656 M×0.010 L=6.56×10-4 mol
Equation 2. nNaOH=CNaOH×VNaOH=0.1008 M×0.0070 L= 7.06×10-4 mol
Equation 3. Theoretical ratio NaOH:HCl from the balanced equation=11=1
Equation 4.Experimental ratio NaOH:HCl=7.06×10-4mol6.56×10-4 mol=1.08
Equation 5. Percent Error=Theoretical-ExperimentalTheoretical100%=2-1.882100%=6.0%
Discussion
The experimental procedure in this lab was relatively simple, employing only a single stage of titration. All the titrations were performed according to the reactions, the stoichiometry of which was investigated. As it can be noted from the percentage error results for each compound pair, the amount of error was overall very small for all of the experiments. In the pair NaOH and HCl the percent error value comprised 9.0% which was the largest error value in this lab. As the lab consisted only of titration, there were no significant limitations or errors residing in the experimental methods or procedures.
For the NaOH: H2SO4 pair the amount of error (6.0%) was lower that for the NaOH:HCl. The results of the experiment are overall accurate and consistent as they agree to the great extent to the theoretical value. The amount of error was lower than for the previous acid-base pair because the volume of NaOH used for titration was greater than in NaOH:HCl. The greater the amount of the solution used in titration, the higher is accuracy because the amount of uncertainty caused by the lab ware is smaller (the ratio of graduation mark volume to the volume used).
The acid-base pairs with Ba(OH)2 had even lower values of percentage error. The Ba(OH)2:HCl pair had only 4.0% difference with the theoretical value. The percentage error for the Ba(OH)2: H2SO4 was even lower and comprised 2.5%.
The theoretical stoichiometry of the reactions was justified by the experiment, and the values of stoichiometric ratios obtained were accurate. The errors were of human origin, and the lab results could be improved if paying special attention to all volume measurements.
Conclusion
In this lab, the theoretical stoichiometric ratio values for four acid-base reactions were experimentally justified. Overall, the purpose of the lab was fulfilled because the experimental values obtained were accurate and the percentage error values compared to theoretical values ranged from 2.5% to 9.0%. The procedures and methods in this lab were simple and possessed no significant intrinsic errors, so it can be concluded that the errors were human errors.
Citations
Bandos, G. (2016). Titration. Chemteacher.chemeddl.org. Retrieved 23 March 2016, from http://chemteacher.chemeddl.org/services/chemteacher/index.php?option=com_content&view=article&id=56
