Experiment 1 Weighing Techniques, Errors, and Statistics Kristele Angeline G. Bilan Joe Vincent E. Cacay Alyssa Lakhsmi M. Maranan Group 1 Bachelor of Science in Chemical Engineering Submitted to the Department of Chemistry University of the Philippines Visayas In partial fulfillment of the course Chemistry 23.1 Under the supervision of Professor Cathrina F. Bagarinao September, 2019 ABSTRACT Using the top loading and analytical balance, samples NaCl, NaOH pellets, ethyl alcohol, and sulfur were weighed separately. Using different weighing techniques, each sample was subject to either weighing by addition, subtraction, error, and statistics. Weights were successfully measured by addition for NaCl(0.58g) and subtraction for NaOH pellets(0.45g), ethyl alcohol(0.18g), and sulfur(0.6g) using their initial and final weights. Weighing errors were measured using factors like temperture, moisture gain or evaporation, and electrostatistically charging before and after the application. Separate weighing was done to evaluate data using statistics. 10 new coins was weighed and the data was used to calculate the mean(5.97199), standard deviation(0.045946), relative standard deviation(0.77%), and confidence interval at the 90% confidence level(5.97199 ± 0.0239) and also underwent the Dixon’s Q-test beforehand to test any outliers. After all data were recorded and compared for final conclusions, it was understood that in weighing samples, one should take into consideration the different factors that affects its weight and the different techniques applicable to the samples. One should also be able to use the different balances in weighing to avoid personal errors and correctly analyze data using statistical instruments. INTRODUCTION To weigh different samples means to have adequate skills in using weighing instruments and in identifying the different balance. Many weighings are rough and require the top-loading balance. However, semi-micro quantities must be weighed using an analytical balance. factors that affect the final weight. Different All measurements contain some procedures should be individually taken into variability so no conclusion can be drawn consideration for any possible errors and the with certainty. Statistics gives us tools to use of different weighing techniques and accept statistics should be evident in acquiring probability of being correct and to reject results. A thorough knowledge and care of conclusions that do not. The mean, is the sum the balances used in the laboratory is of the measured values divided by the essential. The two most common types of number of measurements, n, while the weighing balance used in the laboratory are standard deviation, s, measures how closely the top-loading balance and the analytical the data are clustered about the mean. conclusions that have a high Dixon’s Q test detects a single outlier from a techniques in obtaining the weights of small set of replicate observations (about 3- different 10) in a normal distribution. statistical tests to determine the precision of a This experiment aims to allow the materials and apply simple measurement. students to learn the proper weighing MATERIALS AND METHODS The analytical top-loading balance were balance and cup and the weight was recorded. To obtain used this the weight of the sample, the weight of the in experiment. The materials used in this weighing cupthat containedNaCl was experiment were ten (10) pieces one-peso subtracted by the weight of the empty coins minted in the same year, watch glass, weighing cup. 100-mL beaker, tweezers, weighing cups, test Weighing by difference. The balance used tube, and woollen cloth. EtOH (Ethanol), was a top-loading balance. Three (3) reagents NaCl(Sodium chloride), NaOH (Sodium were used in this test: sodium hydroxide hydroxide) pellets, and Sulfur were the (NaOH) pellets, ethyl alcohol (EtOH), and reagents used in this experiment. Sulfur. The initial weight of the containers Weighing Techniques containing the samples were recorded. Three Weighing by addition. The top-loading (3) NaOH pellets were transferred into a balance was used. An empty, dry, clean clean, dry, pre-weighed watch glass and weighing cup was weighed and the weight weighed immediately upon transfer. Ten was recorded. Three (3) spatulas of sodium drops of EtOH and three (3) spatulas of sulfur chloride (NaCl) were added into the weighing were transferred into two (2) separate clean, dry, pre-weighed weighing cups and weighed EtOH, and sulfur were weighed. The weights immediately. The containers of the samples were calculated using weighing by addition. were weighed again. The weight of the The weights of the samples were then sample by compared to the weight when the samples subtracting the initial weight of the container were first transferred to the watch glass and containing the sample by the weight of the weighing cups. container transferred after was the calculated sample was Effect of electrostatically charged weighing transferred.The watch glass and weighing materials. A clean, dry, plastic weighing cup cups which contained the samples were set was initially weighed and recorded. The aside for some time. weighing cup was then rubbed using a Weighing Errors woollen cloth and weighed again. The Effect of temperature. A test tube holder was weights obtained were then compared to used to hold the test tube. A clean, dry, test know the effect of electrostatically charged tube was weighed. The same test tube was weighing materials. then placed inside a freezer for one (1) minute Weighing Statistics and weighed afterwards. The test tube was The analytical balance was used in then placed inside the oven for approximately this experiment. Ten (10) new Philippine ten (10) seconds and the weight was peso coins were each weighed and recorded. recorded. The values were then arranged from the Effect of moisture gain or evaporation. After lowest to highest and Dixon’s Q test was used a few minutes, the weight of the watch glass to reject any questionable result. The mean, and the weighing cups containing NaOH, standard deviation, relative standard deviation, and the confidence interval of the mean at 99% confidence level was then calculated. RESULTS Table 1. Weighing by addition weight in (g) Empty weighing bottle Weighing bottle + NaCl 1.67g Weight of NaCl 0.58g Table 2. Weighing by difference NaOH EtOH Samples Initial weight of bottle containing 122.82 98.47 the sample Final weight of bottle containing the sample Weight of sample that was dispensed 122.37 0.45 98.29 0.18 Table 4. Effect of moisture gain or evaporation Sample S NaOH EtOH Sulfur Net weight immediately after transfer (g) 0.47g 0.16g 0.57g Net weight after some time (g) 0.54g 0.0117g 1.00g 194.52 Table 5. Effect of electrostatically charged sample 193.92 0.6 Weight of empty watch glass 35.05 1.62 1.64 Weight of watch glass + sample 35.52 1.78 2.21 Weight of sample in the watch glass 0.47 0.16 0.57 Table 3. Effect of temperature Temp. condition Weight of test tube (g) Cold 11.2203g Room temp. (24oC) 11.2131g Hot 11.2130g Weight (g) Initial weight of weighing bottle (g) 1.6822 Weight of weighing bottle after rubbing with woollen cloth (g) 1.6872 Table 6. Weighing statistics Sample coin number 1 2 3 4 5 Weight (g) 5.9861 5.9740 5.9870 5.8910 6.0620 6 5.9948 7 8 9 10 5.9177 5.9710 5.9825 5.9538 clean weighing cup was initially weighed before NaClwas added and weighed again, comparing its initial weight and final weight when it already contained the NaClsample. Another technique used was the weighing by Table 7. Dixon’s Q test for outliers Data Group Suspect Values H: 6.0620 g L: 5.8910 g Qexp Qcrit 0.156 0.466 difference which is an alternative way of computing the mass of a sample reagent. In this method, the difference of the initial For a sample size of 10 at 95% weight of the NaCl container and its final confidence level, the Qexp 0.156 is not greater weight after transferring the sample is the than the Qvalue 0.466, so this point is not at an weight of NaCl.[1] The weight obtained from outlier at an alpha level of 5%. weighing the container before transferring Table 8. Summary of Statistical Results some NaOH increased after it was weighed Statistical parameters Value from the watch glass. Because different Mean Standard deviation, s Relative standard deviation, RSD Confidence interval at the 90% confidence level 5.97199 0.04595 materials were used to hold the NaOH in 0.78% 5.91799 ± 0.0239 5.91799 ± 0.0285 weighed and there was transferring involved, external factors may have affected the weight like moisture gain.[2] Comparing both techniques, weighing by difference proves to DISCUSSION be more accurate because it only involves the In this experiment, we used two difference between the initial weight of the weighing techniques in determining the container before transferring the sample and weight of the samples. The first one is the final weight. It has less to no external weighing by addition in which an empty, dry, factors that can affect the weight like how immediately upon transfer and were set aside moisture gain during the for a few minutes before obtaining their increases net weight of transferring sample weight again. The initial and the final weight reagent.[3]Weighing by difference is used if recorded were different from each other. The one wants to find out the accurate computed weight of the NaOH pellets increased while mass of a sample. While weighing by there was a decrease in the weight of the addition on the other hand can be used to EtOH and sulfur. This is due to the effect of measure after moisture gain and evaporation. [6]As a certain transferring the sample which may be material gains moisture, its weight record affected by external factors and to measure an would have an increase compared to its initial estimate mass of a great amount of sample weight before getting exposed to any external which may be less prone to external factors factors. Evaporation, on the other hand, and in instances that does not need that much reduces weight of a material as any water accuracy. an [4] amount the acquired Since the test tube was exposed content will be lost as gas and will not be to different temperatures and its weight at weighed together with it.[7] Electrostatically different temperatures was recorded, we charged samples or containers can be difficult could see the effect of temperature on the to weigh. Static charges can cause unstable weight of materials. As it was shown in Table balance readings and measurement drift, 3, as the temperature increases, the weight of leading to slow and inaccurate weighing the test tube decreases. It could be concluded results.A negative charge on the material and that the temperature is inversely proportional a positive charge on the balance housing to the weight of the materials. [5] NaOH, EtOH, and sulfur were first weighed cause a force to be exerted between the balance and the vessel. The vertical component of this force adds to the weight of by the observer such as using the right the flask and influences the weighing weighing technique result.[8] accuracy of weighing and by avoiding or that increases the controlling the factors that could affect the SUMMARY AND CONCLUSION measurement. Furthermore, statistical The conduct of the experiment analysis gives us a better evaluation of the showed different techniques that could be data and basis to accept conclusions that have used in the laboratory to weigh different a high probability of being correct and to samples in certain situations accurately reject conclusions that do not. depending on the desired results. It showed the factors considered as common sources of REFERENCES errors which affect the weight of the samples “Electrostatic Charges During Weighing.” Mettler Toledo. Retrieved from https://www.mt.com/ph/en/home/libr ary/white-papers/laboratoryweighing/excellence/electrostaticcharges-during-weighing.html and cause the recorded data to vary such as effect of temperature, moisture gain, and evaporation as well as the material used being electrostatically charged. It is important to note that variations in measurements is normal and no conclusion can be drawn with certainty. In addition, the errors committed by the observer and other systematic errors like environmental and instrumental errors can never be avoided. There are certain methods however to minimize the mistakes and errors committed “Laboratory Weighing.” Chemistry. Retrieved from https://www.cerritos.edu/chemistry/_ includes/docs/Chem_111/Lab/Exp% 203%20Laboratory%20Weighing%2 0Fall%2008.pdf on September 07, 2019. “What Precautions Should Be Taken for Correct Weighing of Laboratory Samples?” Lab-Training.com, LabTraining.com, 22 Jan. 2016, labtraining.com/2014/06/24/correctweig hing-samples/.