Acid/Base Chemistry– Standardizations, Titrations, and a Weak Acid
Student Laboratory Goals:
Over the next lab you will have an opportunity to put much of the theoretical aspects of acid/base equilibria you have learned into practical use in the laboratory. You will apply your knowledge of pH, acid/base indicator, pH electrodes, and titration to solve two research questions.
Research Project:Determination of unknown acid in vinegar via equilibrium constant, Ka, and % by mass of acid.
Student Learning Objectives:
Research Project:
- Apply skills and knowledge of acid/base titrationsto quantitatively solve for molarity of bases and acids.
Q1:What is the molarity of the 0.1XXM NaOH Solution to 3 significant figures
- Standardize a solution of NaOH by titrating a known amount of solid acid, Potassium Hydrogen Pthalate.(Pre-collected data will be offered)
Q2:What is the molarity (M)and % by mass of unknown acid your store bought vinegar? (Assuming the unknown is a monoprotic acid!)
- Titrate an unknown solution of aqueous acidwith the standardized NaOH solution.
- Use calibrated pH electrode for titration, perform three titrations, determine the molarity and % by mass of Acid and average results.
Q3: What is the Ka of the unknown acid in store bought vinegar?
- Use your experimental data to determine Ka in order to identify the unknown acid in store bought vinegar.
For your research project:
- Develop quantitative analytical techniques that are useful in a chemistry laboratory.
- Use a computer interface with pH meter to monitor pH change during titration.
Research Project:
Determination of unknown acid via Equilibrium Constant, Ka
Objectives:
- Perform an acid-base titration using a phenolphthalein indicator and a pH electrode.
- Determine the concentration of NaOH with a known amount of Potassium Hydrogen Pthalate, KHP, otherwise known as standardization.
- Use the standardized NaOH to determine the amount of acetic acid in an unknown.
- Experimentally determine the Ka of the weak acid in vinegar.
Introduction
A titration is a technique used by chemists to determine the concentration of a chemical in solution. Titrations are of fundamental importance to chemists because of their wide range of applications. Specifically, a titration is a procedure for the quantitative analysis of a substance by means of an essentially complete reaction in solution with a reagent of known concentration. In other words, it involves mixing one chemical in a solution of known concentration with another chemical in a solution of unknown concentration to determine the concentration of the chemical in the solution of unknown concentration. For this procedure to be successful several factors must be achieved.
As mentioned above, a solution with a reagent of known concentration must be used for a titration. This solution is called the titrant, and is also referred to as the “standard solution”. For most titrations, the concentration of the titrant is known to three significant figures. The solution being analyzed in a titration has a chemical of unknown concentration that must be determined. The chemical in the solution of unknown concentration is called the analyte. For a titration to be successful, the reagent in the titrant must react completely with the analyte in known stoichiometric amounts. Also, the endpoint of the reaction (sometimes called the equivalence point or neutralization point) has to be clearly identified. To be able to identify the endpoint of a reaction an indicator is generally used. An indicator is a compound with a physical property (usually color) that changes abruptly near the end point of a reaction. The change is caused by the disappearance of analyte or the appearance of excess titrant. In practice, a solution of the analyte is prepared for a titration by diluting it and then adding an indicator. Then the titrant is slowly added to this solution until the endpoint is determined. By knowing the concentration of the titrant and the volume that was added, the concentration of the analyte in the unknown solution can be determined.
In this experiment, you will act as a quality control agent for the Acme Chemical Manufacturing Company. Wiley K. Oty, the new ace vinegar distillery chemist for the company recently developed some new techniques for processing vinegar. Your job is to determine the unknown acid in vinegar and test one of three lots (a,b, or c) to determine the % mass of acid in vinegar in one of these three processes. You will only choose ONE of the three lots and work with that vinegar exclusively!
You will do this by performing a titration in which a sodium hydroxide solution of known concentration will be used as the titrant, the analyte will be a solid sample of potassium hydrogen phthalate, C8H4O4KH(aq), diluted in water, using phenolphthalein and a pH electrode that will determine the amount of acetic acid. By knowing the molecular mass of this compound, referred to as “KHP”, and the amount of sodium hydroxide needed to complete the titration, the mass of the KHP titrated can be determined, and ultimately the purity of the KHP sample.
Q: What is the Molecular Mass of KHP?
A: Potassium hydrogen phthalate, C8H4O4KH, has a molecular mass of:
M.M. of potassium Hydrogen Phthalate = ______g/mol
Sodium hydroxide reacts with KHP according to the following chemical equation:
C8H4O4KH(aq) + NaOH(aq) Na+(aq) + C8H4O4K-(aq) + H2O(l)
Q: Is KHP an ACID or a BASE?
A: ACID / BASE (Please circle one and check with your TA.)
This type of titration would be considered an acid-base titration. KHP makes a mildly acidic solution when it is added to water. The equivalency point of the titration will be when enough NaOH (a base) is added to the solution that it goes from an acidic solution to a basic solution. Phenolphthalein is an ideal indicator for such a titration because it is colorless in an acidic solution, but turns pink in a basic solution. In other words, just as enough NaOH is added to the solution to completely react with the KHP, the phenolphthalein indicator will go from colorless to pink because the solution turns from being acidic to basic. That is to say that phenolphthalein has an enpoint very close to a pH of 7.0.
The stoichiometry indicates that there is a one to one mole ratio between the KHP and the NaOH used. Therefore, the amount of moles of NaOH necessary to titrate a sample of KHP will be equal to the number of moles of KHP in the sample. Once the number of moles of NaOH is known, it can be used to determine the exact molarity of NaOH. Once this titration is performed three times, you may average the molarity of the NaOH and use this solution to determine the amount of unknown monoprotic acid is in your vinegar sample.
You will do three separate titration trials with only a single unknown (a,b or c) and take an average of these trials to determine both the molarity and percent by mass of acetic acid in solution.
Apparatus Set-up – Potentiometric Titration for pH and Electronic Stir-bar Apparatus
Experimental Procedure:
Q1: What is the molarity of the 0.1XXM NaOH Solution to three significant figures?
- Using the weigh boats that have been provided, obtain three ~0.25-0.30 gram samples of KHP and record their masses to the nearest tenth of a milligram. Determine the sample masses “by difference”. In other words, determine the mass of an empty weighboat then put ~0.25-0.30 grams of KHP in it and weigh it again. The difference between the full and empty weighboat is the mass of the sample. Transfer the three different KHP samples to three 250 mL beakers or Erlenmeyer flasks; make sure you can identify each sample with its correct mass! Dilute each sample with 100 mL of distilled water and add 4-5 drops of phenolphthalein to each one. Gently swirl the solutions to completely dissolve the KHP samples.
- Label another beaker as “discarded NaOH” and have it readily available for the duration of the experiment. Attach a buret or utility clamp to a ring stand and set it to a convenient height for holding your buret. Prepare a 50 mL buret in the following manner: Add about 5 mL of the labeled “0.1XX MNaOH” solution (the titrant) to the buret, tilt the buret to a near horizontal position and roll it so the NaOH solution touches the entire inside surface of the buret. Now attach the buret to the ring stand and drain the solution into your NaOH waste beaker. Use a glass funnel to fill the buret with NaOH solution to just above the 0.00mL mark. After the buret is full, keep your eye on the tip of the buret and open the stopcock quickly and drain the solution until no more air bubbles remain in the tip. If the level of the titrant is now below the 0.00mL mark of the buret, record this value as your initial buret reading. If the level is still above the 0.00mL mark, drain enough titrant out to lower the level to below the 0.00mL mark and record this value as your initial buret reading. (NOTE: You must read between the 1/10th mL lines in order to interpolate to the nearest 1/100th mL.)
- Place a white piece of paper on the base of the ring stand and then place your KHP solution on top of this. The paper makes it easier to detect the pink endpoint of the phenolphthalein indicator. Position your buret so that the tip extends into the flask, but you can still easily open and close the stopcock.
- At this timeinsert a cleaned pH electrode, that has been previously washed with deionized water, into your solution, press “Start” on your MicroLAB program, enter “0.00mL” as the initial volume, and press “OK”. Your initial pH reading will be recorded as you begin to titrate your sample. (For further details on the apparatus set-up, please refer to the previous page.)
Proper Titration Techniques:With each addition of NaOH solution, be sure to slowly add and gently swirling the flask to ensure good mixing. This will also ensure that the pH electrode properly equilibrates in between each addition of titrant. During the beginning of the titration the pink color of the indicator will almost immediately disappear. However, as you keep adding titrant the pink color of the indicator will persist for longer periods of time. This is an indication that you are approaching the endpoint of the titration and you should be adding smaller amount of titrant. To reach an accurate endpoint, the titrant needs to be added in very small amounts. This can be achieved by opening the stopcock just far enough to add a drop at a time, or it can be achieved by turning the stopcock 180 degrees as quickly as possible (by doing this your opening the stopcock for a very short period allowing only a tiny amount of titrant out of the buret). When a uniform faint pink color persists in solution for about 15 seconds or longer, you have reached the endpoint of the titration. A brilliant dark pink means that you have probably over shot the endpoint to some extent.
- Be sure to record the final buret reading at the endpoint of the titration. The final buret reading minus the initial buret reading indicates the volume of titrant that was needed to reach an endpoint. The number of moles of NaOH in this volume equals the number of moles of KHP present.
- Continue with the titration past the indicator’s endpoint until the pH of the solution is greater than 11.0. Make your final pH and mL of NaOH reading and then stop the program. Save your data on your N:/ and print out enough copies for you and your labmate.
- Please note that the buret does not need to be rinsed again for the next two titrations, it simply needs to be re-filled to volume that is near the 0.00mL mark on your buret. Repeat the process by completingthe other titration with the known KHP samples that are prepared.
- After all titrations are complete, keep the remaining amount of titrant in your buret and you will be ready for titration of the acetic acid unknown.
- Perform all necessary calculations in your lab notebook to solve for the molarity of your NaOH solution. You should be able to calculate to three significant figures!
NOTE: You have just discovered something about your NaOH solution and have increased the accuracy and precision from ONE significant figure to THREE significant figures. This process is known as “STANDARDIZATION”.
Q2: What is the molarity, M, and % by mass the unknown monoprotic acid in your 10.0 mL of vinegar sample?
- In three cleaned 250mL beakers or Erlenmeyer flasks, use a volumetric pipette to acquire 10.00mL from a single sample of store bought vinegar. Be sure to measure the mass of your 10.0mL solution of unknown acid and record it in the report sheet. Choose from either of the three vinegar samples provided. Dilute your sample to approximately 100mL with deionized water and titrate with your standardized NaOH. (KEEP TESTING THE SAME VINEGAR FOR ALL THREE TITRATIONS.)
- It is not necessary to add phenolphthalein to your unknown acid. The endpoint of this indicator, phenolphthalein, will not directly match the equivalency point of NaOH and unknown acid. You will be using the pH electrode and your graph of pH vs. mL of NaOH to solve for the chemical equivalency point.
- Be sure to acquire pH vs. mL of NaOH data, carefully acquiring data from starting pH to a pH>11.0 for your each of your unknown acid samples.
- For disposal of all titrations for today’s lab, simply add dilute unknown acid, provided in the back of the room, to the beaker dropwise until the pink color of the solution disappears. The solution is now neutral and can be poured down the drain.
Q3: What is the Ka of the unknown acid in store bought vinegar?
Some Calculation Hints:
Remember that molarity has units of mol/L and molecular mass has units of g/mol. Be sure to record mass and volume of your chosen unknown sample of acetic acid on the report sheet. Use the appropriate number of significant figures in your answer.
Look to your course notes on Ka and refer to the constants sheet provided. By the time you have acquired two pH vs. mL of NaOH data, you will have all of the information you need to determine the Ka of the unknown acid.
How will you use these data to experimentally determine this Ka value? (Consider the acid/base buffer region, where pH = pKa.) What is the accepted value for Ka of the unknown acid? (Ka values for weak acids are readily found in a CRC manual OR in a photocopy of the chemistry textbook provided.) Please see your instructor and colleagues for further discussions.
Be sure to discuss in your Conclusions section:
How could you obtain better results?
Are there any flaws in the reasoning of using this technique to determine the molarity of the unknown acid and % by mass of unknown acid?
Name______Date______
Lab Section______Lab Partner______
RESEARCH PROJECT: Determination of Unknown Acid and Ka.
DATA REPORT SHEET
NaOH Molarity:______M
Acid ID and Concentration:Unknown Vinegar Type:______
Scout Titration Trial 1
Mass of 10.00mL Sample - ______g ______g
Initial Volume of NaOH______mL______mL
-Final Volume of NaOH\ ______mL______mL
Total Volume of NaOH at eq. pt. –______mL ______mL
Moles of NaOH at eq. pt. – ______mol ______mol
Moles of monoprotic acid – ______mol ______mol
Molarity of monoprotic acid – ______M ______M
Average Acid Molarity:______M
Ka of Acetic Acid Concentration:
Scout Titration Trial 1
Volume at half-equivalency point______mL______mL
pH at half-equivalence point______
Calculated Ka of sample – ______
Averaged Experimental Ka for Unknown Acid:______
Accepted Ka for Identified Acid:______% error:______
WHAT IS THE IDENTIFICATION OF THE UNKNOWN ACID?
______
Final Calculation! (From your identified Unknown Acid, what is the % by mass?)
% by Mass of identified Acid – ______
Average Mass %:______