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The Identification of An Unknown Acid pKa Melting Point & GMM Lab Report

The Identification of An Unknown Acid pKa Melting Point & GMM Lab Report

Question Description

I’m working on a chemistry report and need an explanation to help me learn.


This exercise asks you to complete the following

  • Title Page
  • Objective
    • What was the goal of this experiment? What data will you use to accomplished it?
  • Introduction
    • Define gram molar mass aka equivalent weight.
    • Define titration. What is the purpose of a titration experiment?
  • What is a primary standard? What was the primary standard in this experiment? (If you didn’t use a primary standard, indicate what it should have been)
  • Why were there 2 standard solutions? That is, why wasn’t NaOH(aq) used as a primary standard for titrating the unknown acid? (If you didn’t use a primary standard, then explain why NaOH(aq) should be used as a primary standard.
  • How would melting of a pure sample differ from melting of an impure sample?
  • What is Ka? What is pKa? What does a high Ka mean? What does a high pKa mean?
  • What is a buffer?
  • For a weak acid, under what conditions will pH = pKa?
  • Procedure Reference
    • What changes were made to the published procedures?
  • Results and Calculations
    • Use the results that were provided to you and clearly indicate your unknown number.
      • Create (don’t copy the Excel tables) a table summarizing your major experimental data (i.e. gram molar mass, melting point, and pKa of monoprotic unknown acid).
      • Show sample calculations.
    • Summarize the gram molar mass, melting point, and pKa of your top monoprotic acids with percent error values. This is probably best done in table format.
  • Discussion
    • State the identity of your unknown monoprotic acid from a… by using
      • Remember to be quantitative, whenever possible.
      • Discuss how you came to that conclusion using your average gram molar mass with standard deviation and average melting point range and pKa to justify your identification.
      • Explain how you narrowed down the suspects and state your second choice and/or third choice, if appropriate and how they were eliminated.
      • Be sure the discussion includes, how many standard deviations the actual molar mass of the acid you believe best represents your unknown is away from the experimental molecular mass of the acid.
      • Also, does the actual melting point of your identified acid fall within the range of your experimental average melting points? Explain why or why not.
      • Discuss the accuracy and precision of your average gram molar mass and melting point. (How do you quantify accuracy? How do you quantify precision?)
    • Discuss any sources of error that may have occurred in the experiment. For example, if you added too much base to one of your titrations, you should identify how that would have affected your gram molar mass calculation. Would it increase or decrease?
  • Conclusion
    • Evaluate the objective of the experiment and restate the identity of your unknown acid, the average gram molar mass, the average melting point range, and the average pKa.
  • Post Lab Questions
  • Spreadsheet
  • Provided Data

Determination of Melting Temperature

  • There are two melting temperature instruments that you should be familiar with. These are both used quite a bit for organic chemistry labs, so it’s best to gain as much experience as you can if you plan on or might eventually need to take organic chemistry.



  • The following video provides some good tips for determining melting temperature and provides a nice example of how to take the measurement.

Calibrating and use of a pH probe

To determine pKa of a weak acid a calibrated pH probe is required. Watch this video to see how a Vernier LabQuest2 can be used to calibrate a pH electrode. In the video, only 2 calibration standards are shown. There are 3 common pH standard solutions, pH 4, pH 7, and pH 10. It is common to use the two standard solutions that are closest in pH to the solution(s) that will be measured.

When the probe is not being actively used, it should be stored in the storage solution. The electrode should not be allowed to dry.

After calibrating the electrode, it can be used to measure the pH of samples.

When all measurements are complete, the electrode should be placed in storage solution. The storage solution is usually not merely deionized water.


Use the data that was provided to you to determine the molar mass of the unknown acid. You should have completed a similar calculation when you did your prelab quiz.

  • First calculate moles of NaOH that were needed to neutralize all the acid.
  • Use stoichiometry to determine how many moles of acid must have been present in the titration experiment.
  • Use the mass of the acid and the moles of the acid present in the titration to determine the molar mass
  • Calculate the molar mass for each trial
    • Calculate an average and a standard deviation


Follow the link that was provided to you with the rest of your data. Assume that the video represents the same unknown that was analyzed in the titration.

  • Watch the video and record in your notebook an appropriate melting point for the unknown substance. Remember how you are supposed to report the melting temperature based off of the training in the prelab.
    • The thermometer in the video measures temperature in oC.

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