Experiment 7 Lab Report PDF

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Michael Arnold Lab Partner: Jacob Porter TA: Ting Wang November 20, 2016 Experiment 7 CHE 111-048 Determining the Concentration of a Solution Introduction: The purpose of this experiment is to be able to titrate a known solution in order to determine the concentration of that known solution. The calibration of the pH probe is very important because the pH readings need to be very precise in order for calculations and dimensional analysis to be accurate. The calibration can be done by putting the probe in known pHs of solution. This allows all readings for the rest of the experiment to be accurate. NaOH(aq) + KCH8H4O4(aq)  NaKC8H4O4(aq) + H2O(l) This is the reaction between KHP and NaOH. This is needed for the initial titration to get the standardization of the NaOH. This means finding the molarity of the NaOH by using a known concentration of KHP. The molarity of the KHP will be calculated converting the measured mass to moles. This can then be divided by the liters of solution in order to get the molarity of KHP. This can then be used to calculate the molarity of the NaOH. The amount of NaOH that is needed to get to the equivalence point for the KHP can be used, along with the KHP molarity, to find the molarity of NaOH through dimensional analysis. NaOH(aq) + HC6H7O7(aq)  NaC6H7O7(aq) + H2O(l) This is the reaction of the citric acid and NaOH. This is needed in order to calculate the molarity of the citric acid. This can be done using a known concentration of NaOH, which has already been calculated. The volume of acid, along with the known concentration of NaOH, and

the amount of NaOH needed to get to the equivalence point can be used to calculate the molarity of the citric acid though dimensional analysis. A titration works because it allows a known concentration of solution to find the concentration of the other solution. The buret allows for a constant rate of a solution to react with another solution. As the two solutions continue to react, the equivalence point will eventually be reached. This means that the number of moles of each solution is equal to each other. With this equivalence point, you can calculate the molarity of the other solution.

s=

√

2

Σ (x−avg ( x ) ) n−1

The above equation is the equation for standard deviation. This is an equation that helps show how far apart the points of measurement are spread out away from the average of the set of numbers. The smaller the value for standard deviation, the closer together the set of measurements are, which is good. The larger this number is, the farther the set of measurements is from the average of the set, which is not good. The hypothesis of this experiment is that the standard deviation of the measurement points will be a very low value. This means that the measurement points will be fairly close to each other. Materials: The materials can be found on page 98 of the lab manual (French, 2015) Procedure: Follow the procedure as stated on pages 98-101 in the lab manual (French, 2015). Changes: 15.) Instead of 150 mL, obtain 200 mL of NaOH 41.) Only need two trials of KHP and NaOH; not three trials

47.) Only need two trials of NaOH and citric acid; not three trials Discussion: The purpose of this experiment was to be able to titrate a known solution in order to determine the concentration of that solution. The purpose was met because a titration was performed. As a result, the concentration of the solution was able to be determined. The hypothesis of this experiment was that the standard deviation would be a relatively low value. This hypothesis was confirmed after calculating what the standard deviation actually was. After conducting the multiple titrations, it was calculated that the average molarity of the NaOH was 0.192  0.001 M. This was calculated using the molarity of the KHP solution, and the amount of NaOH that it took for the KHP solution to reach its equivalence point. For the second set of titrations, it was calculated that the average molarity of the citric was 0.0749  0.00071 M. This was calculated using the average molarity of the NaOH, and the amount of NaOH that was needed for the citric acid to reach its equivalence point. These results do support the purpose because a concentration for both the NaOH and the citric acid solutions was able to be determined. These results also support the hypothesis because the standard deviation was very minimal for both the NaOH and the citric acid molarities. There are multiple possible explanations for why the standard deviations wouldn’t have been zero, meaning there were no variations in the results. The first is that when reading a graduated cylinder or a balance, the equipment can only be accurate to a certain degree. The balance for example only goes to the fourth decimal place. This is not something that can be prevented necessarily other than buying more accurate equipment or just making sure that the

equipment is being read as accurately as possible. Another explanation would be if the pH probe wasn’t calibrated perfectly. This would cause the equivalence points to be inaccurate and would cause the calculation of the concentrations to also be inaccurate. In order to prevent this, it is extremely important to correctly calibrate the probe to be as accurate as possible before starting the actual experiment. One last possible explanation for a source in error would be if the rate of flow of the titrant was not constant. This could have caused the MeasureNet to get inaccurate readings and thereby resulting in an inaccurate calculation of concentration for the solution. To prevent this from occurring, it is important to open the stopcock to a flow that is not too fast or too slow, and then leaving it at that rate. Conclusion: For this lab I felt as though I was able to learn a lot. This was my first experience with a MeasureNet system. I had done titrations in high school, but not with a system and probes that were capable of what the MeasureNet does. Although I had done titrations in high school, this was my first real experience of doing the calculations that go along with a titration. The only realistic suggestion I have for improving this lab would be to tape the probe to the inside of the beaker while doing the titration. I found that although the probe was resting in a holder in the drop counter, it still moved when swirling the solution and this caused inaccurate readings for some titrations. By taping the probe to the inside of the beaker, it keeps it in place while swirling the solution. Works Cited: 

French, April, Allison Soult, Stephen Testa, Pauline Stratman, M. Meral Savas, Fancois Botha, Carolyn Brock, Charles Griffith, Darla Hood, Robert Kiser, Penny O’Connor, William

Pluncknett, Donald Sands, Diane Vance, William Wagner. “CHE 111 General Chemistry Laboratory Manual.” 2015-2016 Edition. Pages 81-86 Print....