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Experiment 5

Experiment 5. % Potassium Hydrogen Phthalate (KHP) in an Unknown. Writing Lab Reports. See Course Website for “Academic Integrity” and “Writing Lab Reports” Handouts. 6 sections. Title Introduction Procedure or Methods Results Discussion or Conclusion References. Title.

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Experiment 5

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  1. Experiment 5 % Potassium Hydrogen Phthalate (KHP) in an Unknown

  2. Writing Lab Reports See Course Website for “Academic Integrity” and “Writing Lab Reports” Handouts

  3. 6 sections • Title • Introduction • Procedure or Methods • Results • Discussion or Conclusion • References

  4. Title • Same as in notebook • brief statement about what is to be determined and how. The title should be descriptive enough that you do not need the “purpose” section that you may have written in high school. DO NOT COPY THE HANDOUT TITLE AS IT MAY NOT BE DESCRIPTIVE

  5. Introduction • NEW • explains the chemical principles behind the experiment. • It should explain what you are trying to determine by performing the experiment • explain why you are using the chosen method. • Explain the chemistry of why the method allows you to determine what you are looking for. • It is often helpful to give the chemical reactions and EXPLAIN what is happening at each step. • Third person present tense • Best to prepare before performing the experiment.

  6. What should the intro for this experiment include? • What is the reaction? • Be sure to explain the reaction instead of just drawing it out. What types of chemicals are reacting with each other and what are they forming? • What is the stoichiometry? • Important because it is used in calcs • Differences between primary and secondary standards • What are the conditions for the system at the endpoint? • What was used to indicate when to stop? Why was it chosen? • How the titration allows for you to find %KHP in the unknown

  7. Be sure to answer all of these questions in intro • What happens during a titration? • Define primary standard. What was your primary standard? • Define secondary standard. What was your secondary standard? • What type of reaction is being evaluated? What types of reactants are being used? • What is the specific reaction that is occurring? What is the stoichiometry of the compounds that you need to consider? • What is the condition of the system at the end of your titration? • What tells you when you have reached the end of this titration? • What properties of this particular indicator made it a good choice for this titration? • How does the amount of NaOH tell you the %KHP in the unknown?

  8. Method/Procedure • Similar to notebook • Complete sentences and paragraphs • Third person past tense

  9. Results • Similar to notebook • All in Computer-generated Tables and graphs/charts • Tables/Graphs labeled with “Table/Graph #” & Descriptive Title • All paired data kept together • All values have units • All values have appropriate significant digits • Important observations can be paired with the appropriate results

  10. Discussion/Conclusion • Same as notebook • I will be looking very heavily at your logic for errors and conclusions • Must be third person • Well structured, grammatically correct paragraphs – not just a list of results

  11. References • MLA/APA/CBE format (learned in English classes) • Cited within the appropriate section of the text of the report, as well

  12. Experimental Goals • To prepare a carbonate-free solution of ___________ • To determine the _________ of the NaOH solution by titration (called standardization) • To determine the ____ in a solid mixture using the standardized NaOH solution

  13. Titrations • Slow addition of reagent of known concentration is added to analyte to complete reaction in order to determine the amount of analyte present • When you titrate something you are taking it to the equivalence point

  14. How do you add slowly and know exactly how much you added? • By using a Buret • Reading a Buret

  15. Equivalence Point vs. End Point • End point- • Equivalence point-

  16. LeChatelier’s Principle • LeChatelier’s Principle- when a stress is added to an equilibrium system, the system will shift to remove the stress. • Before titrating, we have acid in water: HA  H+ + A- • For this reaction, the stress is the addition of OH-. OH-+H+  H2O. This removes H+ ions from the right side of the initial reaction, making the equilibrium offset. Therefore, the equilibrium will shift to the right (making more H++A-) to release the stress. • It is the equivalent of putting a bunch of children on a teeter totter. If it is balanced (in equilibrium) and one child gets off from the right side, a small child could leave the left side and go to the right side to make the teeter totter balance again.

  17. So, how do we determine our End Point? • Consider the reaction taking place • Start with weak acid: HA • In water it has an equilibrium: HAH++A-conj. base • Add strong base: HA+NaOH  H2O+A-+Na+ • When enough base (an equivalent amount) has been added to use up all of the HA (in the form of H+), you are left with just A- and Na+ in solution. The HA has been neutralized. This is due to the LeChatelier shift. • The A- reacts with H2O: A- + H2O  HA + OH- • This will make the solution slightly basic • If you add one more drop beyond this, your solution will be extremely basic (dependent on the pH of the solution that you are adding)

  18. The Titration All A- in H2O Buffer Region HA + OH- A- + H2O A- + OH- ACID BASE

  19. End Point? • Summarize Reaction (What is happening?) • Start • End Point? • Choose an appropriate indicator • Our end point is

  20. At first, not much. As more NaOH is added, you will see pink where it is added. The pink color will fade As you get closer to the end point, the color will remain longer At the end point, the whole solution will turn pink. You want it to be pale. If you are not sure that it is pink, record volume and add one more drop to test What will we see?

  21. Observations??????

  22. Hazards • Phenolphthalein • KHP • NaOH

  23. Primary vs. Secondary Standards • 1° = a reference standard of known purity (often a dry solid). You can know EXACTLY how many moles you have (by knowing mass and molecular weight) • 2° = not initially sure of concentration. You use the 1° standard to determine concentration in order to use this standard to determine amount of something else. (this is our NaOH solution)

  24. Calculations • Want to determine Molarity (moles/L) of NaOH • At the end of Exp’t. you will know: • Mass (g) KHP used “x” • Volume (mL) NaOH titrated “a” *(convert to L by * by 1L/1000mL) • mw KHP (204.22g/mol) from handout • Molar ratio: 1molKHP:1molNaOH

  25. Reminder:Tables in Notebooks • Put DESCRIPTIVE titles on all tables • Pair data and results for each trail together • It is easier to find and compare data/results if all paired data is kept together in tables with descriptive titles

  26. Experimental Design • Variables • What type of standard to use • What type of indicator to use • How much solid to weigh • Concentration of NaOH • Critical Measurements/Steps • Weighing KHP • Reading buret • Seeing the end point • stopping titration

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