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Finding Concentration of a Solution

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Introduction

Lab 1.5 Finding Concentration of a Solution Jason Yu (Block C) Research Question Which of the following methods give a value of NaOH concentration closest to the 'real' value1? 1. Weighing the solute, 2. Titrating it 1. No literature value available, we need to find by most accurate method Data Collection: Method A (Weighing NaOH) Quantitative Data: Method A: Weighing NaOH Mass of NaOH pellets (zeroed beaker) = 0.412 � 0.001 g Volume of standard solution = 100.0 � 0.5 cm3 Qualitative Data: On electronic balance: Two white and round NaOH pellets which were dry soon become wet and moist on surface, as seen by it gaining luster at time of measurement, which was a minute or two after taking out of the storing bottle.. In preparing NaOH solution: By crushing with glass rod, white NaOH pellets dissolve in water to form clear solution. Data Collection: Method B (Titration) Quantitative Data: Method B: Titration of NaOH Titration No Rough 1 Rough 2 1 2 3 4 (Vanessa Yu's) Indicator (titration by) phn1 phn phn phn phn phn Final burette reading (cm) 9.9 11.80 30.4 30.5 39.9 42.4 Initial burette reading (cm) 0.1 2.25 21.0 21.0 30.5 32.9 Titre (cm3) 9.8 9.55 9.4 9.52 9.42 9.5 1 Phn = Phenolphthalein acid-base indicator. 2 Around � 9.40 to � 9.45 3 Only the titres in bold will be used in calculation Precision of burette readings = � 0.1 cm3 Volume of pipette = 10.00 � 0.04 cm3 (class B) ...read more.

Middle

Conclusion: From our qualitative data, as we will see that in Method A a major error occurs, where NaOH absorbs water on the balance, displaying a greater weight than it should be, while Method B has no major error except for minor manipulative errors which have been as avoided as possible. Therefore Method B is more accurate than Method A. Explanation: We have assumed that the weight recorded of NaOH on the balance agrees with the real value of the pellets, i.e. there is constant mass of NaOH during the measuring of weight. However, it is known that NaOH is a hygroscopic substance, i.e. it will readily absorb water from the air, which explains why we see the NaOH become moist and show watery luster on the balance. This makes the value of mass obtained larger than it is without the water absorbed, thus making the mass recorded inaccurately showing a larger value. This will give us a larger calculated concentration (as M = (mass / molar mass) / V), which agrees with our final data, where Method A gives a concentration of our standard solution to 0.103 M, which is larger to the concentration found by the second method, which gives 0.0977 M. Thus method A is more inaccurate by this large systematic error. The probable accuracy of Method B is higher, as its main errors are due to manipulation error, and also small random errors which both errors do not contribute much to affect the results in our experiment, as I have been very careful with a major source of error which could affect the titration. ...read more.

Conclusion

We could of course do the experiment in an environment absent of water vapor if we have to, but it would be had to control the water vapor level in environments. As for Method B, we should use a burette which has markings on front and back so you are know you are reading at eye level, thus reducing measurement inaccuracy. We could increase the precision by using more NaOH (more than 10 cm3) to get 3 sig. fig.s, as we are getting 9.4 now, which is just a 2 sig fig. value, in case even trial repetition does not increase the number of sig figs. We should titrate fast until the volume used reaches something close to the rough value we did, and titrate drop by drop, to make sure the time we used in the titration was not too long and that the end point was not passed. At the same time of controlling the tap, we should shake the flask thoroughly. Some drops of HCl may not reach the NaOH but instead be on the sides of the flask instead. Make sure no drops fell to the outer parts of the flask or the mouth of it. Make sure that the tip of the burette hangs a drop before and after the experiment so that it is not counted as it didn't go into the flask. Ensure that there are no air bubbles trapped inside before doing the titrations. Do more trials to improve both accuracy and precision of the titration. ...read more.

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