Physical Chemistry Experiment Analysis - Lab Report Example

Partner’s TA Experiment Unknown KHP Determination Sample ID Introduction The purpose of this experiment is to accurately and precisely determine the concentration of sodium hydroxide (NaOH) solution, and subsequently determine the mass percentage of KHP in an unknown sample. Technically, the concentration of sodium hydroxide solutions is inaccurate when mass of NaOH is used in deducing the solution’s molarity. Errors resulting from the inaccuracy of concentration will compromise on accuracy of subsequent experiments. In quantitative experiments, the scientific concepts of accuracy and precision are instrumental in determining reliability and validity of findings (Stanton, 26). Accuracy refers to the degree of technical intimacy between actual measurement and a known standard measurement. On the other hand, precision is concerned with the degree of reproducibility in measurements. Admittedly, both accuracy and precision are key determinants of reliability. In the presence of errors like the inaccurate molarity of NaOH, measurements in subsequent procedures are usually compromised. This lab report covers on the determination of KHP in an unknown through standardization of sodium hydroxide solution using potassium hydrogen phthalate, commonly abbreviated as KHP. NaOH is not only highly hygroscopic but also absorbs carbon dioxide from air. These properties of NaOH yield substantial errors in the compound’s mass measurements. In practice, actual concentration of NaOH is determined using KHP as an acidic primary standard (Stanton, 32). The equation for the reaction is KHC8H4O4 + NaOH → NaKC8H4O4 + H2O. KHP has one H+ whereas NaOH has a single OH-. In ionic basis, H+ in KHP reacts with the OH- in NaOH to form H2O in the ration 1:1. By titrating a weighted amount of KHP against NaOH, the actual molarity of NaOH can be derived from the resultant titration data. Once the molarity of NaOH has been determined, the percentage purity of an impure primary standard can be deduced. A suitable indicator, which in this case is phenolphthalein, was used to indicate the titration’s end-point. Method Preparation of NaOH Solution Theoretically, standardization of NaOH and determination of a primary standard’s purity are quantitative methods. Therefore, accuracy of instrument readings is paramount in ensuring success of a volumetric analysis. In this case, the experiment began with the cleaning and calibration of instruments, primarily the burette, mass scales and Erlenmeyer flasks. Once the instruments were ready, a solution of NaOH was prepared. Procedurally, preparing the solution of NaOH is the first step in determining the actual molarity of NaOH, which will be used in determining the percentage purity of KHP. Slightly concentrated NaOH reacts actively with CO2 to form carbonate ions, a weak base which compromise subsequent steps of a titration experiment (Stanton, 32). Therefore, a highly concentrate NaOH was used to make the solution. In addition, distilled water was boiled for approximately 10 minutes to eliminate any dissolved CO2. Finally, 3ml of 50% NaOH was dissolved into 1000ml of cool water, thus yielding approximately 0.1M NaOH solution. Standardization of NaOH Solution Once a solution of NaOH was prepared, a weighed amount of KHP was used in deriving the actual concentration of the base. A relatively pure sample of KHP was dried at 110oC to eliminate moisture. At this juncture, is should be acknowledged that the standardization sample of KHP is different from the unknown sample of KHP. Three trial samples of standard KHP were measured and diluted with 50ml of distilled water. In order to prevent the effect of environmental factors, one sample of the solid KHP was dissolved and titrated against NaOH in a burette at a time; while the remaining two samples awaited dissolution. Mass of KHP solid in each titration and a corresponding volume of NaOH dropped from the burette were recorded in the data table and used in determining the concentration of NaOH. Determination of Unknown KHP Finally, the determined concentration of NaOH was used in analyzing the unknown KHP sample. The unknown KHP sample comprised of two compounds, potassium hydrogen phthalate and neutral diluents. In this step, the determined molarity of NaOH was used in deducing the percentage purity of the unknown KHP. Three samples of the unknown KHP sample were weighed and dissolved with 50ml of distilled water, each sample in a separate Erlenmeyer flask. Two drops of phenolphthalein indicator were put into each Erlenmeyer flask, and the KHP solution titrated against NaOH. The mass of unknown KHP sample and the corresponding volume of NaOH used were recorded in a data table. Subsequently, calculations of NaOH molarity and percentage purity of the unknown KHP were performed. Data and Results Table 1: Standardization of NaOH Trial Mass of KHP (g) [NaOH] mol-L-1 1 0.7114 0.0713 2 0.7627 0.0707 3 0.7428 0.0712 Average = 0.0711 mol-L-1 In deriving the molarity of NaOH, the moles of KHP in each sample were reconciled with the volumes of NaOH in each standardization burette reading. Since the stoichiometric ratio of the acid and base is 1:1, then the moles of NaOH was easily determined based on the moles of KHP in each trial. On average, the concentration of NaOH to be used in subsequent step was 0.0711 M. Table 2: Unknown Data Trial Sample mass (g) Volume of NaOH (ml) Mass % of KHP 1 1.6055 46.7 49.00 2 1.6094 47.2 49.40 3 1.6159 47.4 49.41 In the 1st trial, 46.7 ml of 0.0711 M NaOH was used. In this case, moles of NaOH that reacted with the trial 1 KHP sample is given by; (0.0711×46.7)/1000 = 0.00332 moles of NaOH. Similarly, 0.00322 moles of KHP reached with the base. Molar mass of KHP = 204.233. Therefore, 0.00332 moles of KHP = 0.6781g. In percentage form, mass of KHP in the unknown KHP sample is given by (0.6781/1.6055) × 100. Percentage of KHP by mass = 49.00%. Similar calculation procedure was used in deriving the percentage mass of KHP in trials 2 and 3. On average, the percentage mass of KHP in the unknown sample was 49.27%. In order to improve certainty in the percentage of KHP within the unknown sample, a confidence interval was calculated using the formula u = x +/- ts/√n. At 95% confidence level, the value of t at n-1 = 4.303. X (mean) = 49.27 % while s (standard deviation) = 0.191. Therefore, the confidence interval value is given by (4.303×0.191)/√3 = 0.26. At 95% confidence limit, the percentage mass of KHP in the unknown sample is 49.27 +/- 0.26. Discussion and Conclusion At this juncture, it emerges that the there is 49.27 +/- 0.26 % of potassium hydrogen phthalate in the unknown sample. The remaining percentage in the sample is made up of the neutral diluents. Technically, use of confidence interval in reporting experiment results enhances the aspect of certainty and reliability (Stanton, 49). For example, there is a 95% probability of finding the average percentage mass of KHP in the sample within the range of 49.01-49.53%. Actually, the confidence interval incorporates a sense of probability in the results, but narrows down the probabilities to a deterministic range of values. Lack of a finite accuracy in the results can be attributed to possible errors in the experiment. One likely cause of errors in the unknown KHP determination experiment is instrument resolution. The smallest division of burette scale is 0.1mm. In this case, it is not possible to make precise measurements exceeding one decimal place. Undeniably, the absence of finite precision in instruments yields errors of considerable magnitudes, especially in quantitative methods (Stanton, 35). Another probable cause of errors in the experiment is environmental factors. In the experiment, KHP samples had to be dried to eliminate moisture while the effect of CO2 in NaOH solutions was prevented to a relatively high degree. However, there is a remote yet distinct possibility that CO2 and moisture in the lab environment affected the results in an infinitesimal but significant manner. Therefore, the possible effect of errors was neutralized by the use of confidence intervals in reporting. In conclusion, it is evident that procedural execution of a quantitative volumetric analysis yield reliable findings. Work Cited Stanton, Bobby. Experiments in General Chemistry: A learning-based approach. Pittsburg: Cengage Learning, 2009. Print. Read More