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The primary goal of this laboratory experiment is to identify an unknown weak acid through a systematic approach involving titration and various chemical analyses. The experiment will include calculations, formulas, and tables to support the identification process.
Materials:
Procedure:
Add 25.00 mL of the unknown weak acid solution to a beaker. b. Add a few drops of phenolphthalein indicator to the beaker. c. Titrate the unknown weak acid solution with the standardized NaOH solution until a persistent pink color appears. d. Record the volume of NaOH used in the titration.
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b. Since the reaction is 1:1 between NaOH and the unknown weak acid, moles of NaOH equal moles of the weak acid. c. Calculate the concentration of the unknown weak acid.
b. Create a table listing potential weak acids and their respective pKa values. c. Identify the unknown weak acid based on the closest match.
Based on the experimental data and analysis, the unknown weak acid can be identified by comparing its pKa value to known pKa values for various weak acids. The confirmation tests provide additional support for the identification. The systematic approach of titration, pH measurement, and comparison with known data enhances the reliability of the identification process.
This is a general outline, and the specific details of calculations, formulas, and tables may vary depending on the actual experimental conditions and the properties of the unknown weak acid. Adjustments to the procedure may be necessary based on the specific characteristics of the unknown acid and available equipment.
Acid-base titration is a fundamental technique for determining the properties and characteristics of acids or bases. This method involves a neutralization process that offers insights into the molecular mass and pKa values of the substances involved. Monitoring the endpoint of the titration can be achieved through various means, including the use of indicators or, as demonstrated in this experiment, a pH meter equipped with electrodes.
A titration curve serves as a graphical representation of measured pH values obtained from the pH meter readings against the volume of titrant added in milliliters. The key objective of acid-base titration is to identify the equivalence point, which is theoretically achieved when the moles of base added into the Erlenmeyer flask equal the moles of the acid. This point is characterized by a significant and noticeable change in pH with a relatively small change in the volume of the titrant.
For this experiment, an unknown monoprotic acid was targeted for identification through titration with an approximately 0.1 M sodium hydroxide solution. A 10-milliliter sample of the unknown acid was titrated, and three trials were conducted, resulting in three distinct titration curves.
The obtained pKa and Ka values from the titration process were determined to be 4.537 and 2.90 x 10^-5, respectively. Additionally, the initial pH of the unknown acid was used to derive alternative pKa and Ka values, yielding 4.81 and 1.53 x 10^-5, respectively. The theoretical values for acetic acid, believed to be the unknown acid, are 4.75 for pKa and 1.76 x 10^-5 for Ka.
Comparing the experimentally derived values to the theoretical values, the titration process exhibited an error of 4.48% for the pKa value and 64.77% for the Ka value. In contrast, the initial pH method demonstrated a much smaller error, with 1.26% for pKa and 13.1% for Ka. These results indicate a slight difference from the theoretical values, emphasizing the initial pH method's superiority in accuracy.
In conclusion, the unknown monoprotic acid has been successfully identified as acetic acid through the conducted experiment. The comparison of values derived from the titration process and the initial pH method revealed that the latter provides more accurate results, with significantly lower errors compared to the theoretical values. This successful identification highlights the importance of choosing an appropriate method in acid-base titration experiments for precise and reliable outcomes.
The primary goal of this experiment is to identify an unknown monoprotic acid by comparing calculated average Ka values with the Ka values of common acids. The determination of Ka values involves titration processes based on pH and the initial pH of the unknown monoprotic acid.
This experiment focuses on monoprotic acids, where an acid acts as a proton donor and a base as a proton acceptor, as per Brønsted and Lowry. Monoprotic acids donate one proton, while polyprotic acids donate more than one. Monoprotic acids release one proton per molecule, leading to a single equivalence point.
When a weak monoprotic acid (HA) dissolves in water, only certain molecules dissociate to yield hydronium ions (H3O+) and A- ions. The equilibrium reaction is represented as HA (aq) + H2O (liq.) <−> H3O+ (aq) + A- (aq).
The ionization constant (Ka) characterizes the acid and is calculated using the equation Ka = [H3O+] [A-] / [HA]. Strong acids have Ka values greater than 1, while weak acids, like propanoic acid with a Ka of 1.3 x 10^-5, have Ka values much smaller than 1. Scientists often use pKa values for convenience.
The number of moles of acid can be determined using the volume and concentration of the base (sodium hydroxide) with equations ηbase = massbase / Mr base and ηacid = ηbase x 1 mol of acid / 1 mol of base.
The pH of a solution is related to H3O+ concentration by pH = −log [H3O+]. The pKa of an acid is pKa = −log Ka. Substituting into the ionization constant equation, pKa = pH – log [A-] / [HA].
At half-equivalence point, pH = pKa + log [A-] / [HA].
The experiment utilizes a 50-millilitre burette, 10-millilitre graduated cylinders, 50 and 250-millilitre beakers, Erlenmeyer flask, retort stand, burette clamp, pH meter, spatula, analytical balance, filter funnel, magnetic stirrer, pellets of sodium hydroxide, distilled water, and 10 millilitres of unknown monoprotic acid.
EXPERIMENTAL PROCEDURE
The experimental pKa and Ka values for the unknown acid are determined to be 4.537 and 2.90 x 10^-5, respectively. Additionally, using the initial pH of the unknown acid, the calculated pKa and Ka values are 4.81 and 1.53 x 10^-5, respectively. The theoretically expected pKa and Ka values for acetic acid, the believed identity of the unknown acid, are 4.75 and 1.76 x 10^-5.
An analysis of the data reveals an error of 4.48% in the pKa value and 64.77% in the Ka value obtained from the titration process. Meanwhile, using the initial pH results in an error of 1.26% in pKa and 13.1% in Ka values. Despite the slight differences, the values obtained from the initial pH method are closer to the theoretical values compared to those from the titration process.
Based on the results and analysis, the unknown monoprotic acid is identified as acetic acid. The values determined using the initial pH method are deemed more accurate than those obtained from the titration process.
Several recommendations and precautions can enhance the accuracy of the experiment:
Ensure the standard solution used is 100% pure and stable at room temperature. Drying the standard material before weighing and dilution is preferable.
To strengthen the identification of the unknown monoprotic acid, consider determining its molecular weight. Obtain the mass of the solid acid before dilution and titration, allowing for a comparison with the theoretical molecular weight of acetic acid.
Identification of Unknown Monoprotic Acid: Comparative Analysis of Titration and pH Methods. (2024, Feb 26). Retrieved from https://studymoose.com/document/identification-of-unknown-monoprotic-acid-comparative-analysis-of-titration-and-ph-methods
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