You are the TA for the previous experiment (titration of HCI with NaOH), and the

Question

You are the TA for the previous experiment (titration of HCI with NaOH), and the following data was collected for the molarities of HCI, and the following interpretation of the data was handed into you. The student hypothesized that their experimental values for the molarity of HCI would be approximately 0.2 M for all of their trials because the same amount of acid was being titrated each time with the same concentration of base. What feedback would you give to the student in regards to their presented conclusions and data, and what suggestions would you make so that they could hand in a better post-lab for the next experiment? 4) Trial 1: 0.3104341 M Trial 2:0.2930437 M Trial 3: 0.3084172 M Trail 4: 0.3021748 M -Values of 0.310434, 0 2930437, 0.3084172, and 0 3021748 M are accurate but not precise, and disagree with my hypothesis by 0.1 M. The imprecision is likely due to different amounts of water being used to wash the acid into the flask each time. In the future, more precise results could be obtained by adding the same amount of water each time. Critical Thinking Question - Please use one to three concise sentences to answer this question, which should be more than sufficient. Keep in mind these are potentially real situations that we have encountered in labs in the past. A student is performing a titration of acid with base, and has properly prepared all solutions (including remembering to add phenolphthalein to the acid). Part way through their titration, they find that their buret has run empty because this is their 4th trial. What should the student have done to prevent this and what should they do now? 5) 154

Explanation / Answer

4) The experimental results are more or less precise, being close to 0.3000 M. The student could have determined the standard deviation of the results. An excel program easily computes the standard deviation of the given set of values. The standard deviation (using an excel program) turns out to be 0.007818 M.

The results are however grossly off from the accepted or desired value of 0.2 M (or 0.1 M; two different values are given). The student obtained the molarity of HCl to be much higher than the desired value. The student didn’t offer an explanation as to why the results were significantly higher than the desired value. This is a gross systematic error and can stem from the fact that the student didn’t do proper dilution of the supplied solutions. Another source of error which may produce significant deviation is wrong calculation of the concentration of NaOH standard. The student should include potential sources of error, since his values were off by a large margin.

5) The student should have filled the buret to the zero mark before each trial This would have ensured that the student didn’t run out of the titrant half way through the titration. The only way to make the experiment work in such a situation is to correctly identify the volume of titrant already added, fill the buret with the same solution of the titrant upto the zero mark and run the titration till the end point. The volume of the titrant consumed will be the sum of the volumes during the two runs. However, care must be taken that the same solution of the titrant must be used in both the runs; else the calculations will go haywire. Moreover, the buret must be re-filled with the titrant as quickly as possible. Time delays may lead to unwanted side reactions. This is especially true when carrying out redox-active titrations.

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