Ionization Constant of a Weak Acid (HA)
- To determine the ionization constant (Ka) of a weak monoprotic acid (HA) by titrating it against a standardized base.
HA(aq) + H2O(l) <==> A─(aq) + H3O+(aq)
- To determine the concentration (M) of the unknown acid HA.
- To learn to use an electronic drop counter to set up an automated titration with high accuracy, and to use Loggerpro software for analysis of titration data. (This 3rd objective is not achievable with the current virtual lab.)
A sample solution containing a weak monoprotic acid, HA, of unknown concentration is titrated against a standardized base (NaOH). The titration data of solution pH versus volume of base added are plotted in real time using Loggerpro software to generate a titration curve. Data from the titration curve are then used to determine the ionization constant (Ka) and the initial molarity of the undilute acid sample.
There are two parts to this experiment:
Part A: Prepare a 200-mL solution of standardized NaOH which will be used to titrate a sample of a weak acid. NaOH is very hygroscopic, meaning that it readily absorbs moisture from the air, and therefore is difficult to measure the dry mass precisely. As a result, a prepared NaOH solution needs to be standardized against a primary standard to obtain its exact molarity. Starting with a NaOH stock solution of ~10 M concentration, you will prepare 200 mL of a dilute NaOH solution and titrate it against a primary standard of potassium hydrogen phthalate (KHP). From the titration data, you will determine the exact concentration of the NaOH solution.
Part B: Use the standardized NaOH prepared in Part A to titrate a sample of a weak monoprotic acid. From the titration plot, you determine the ionization constant and the initial concentration (in M) of the acid sample.
Part A: Standardization of a Dilute Solution of NaOH
1. Preparing a 200-mL dilute solution of ~0.1 M NaOH
You are supplied with a stock NaOH solution of approximately 10 M concentration. This is too concentrated to use directly in a titration experiment. You need to prepare a 500-mL dilute solution of ~0.1 M NaOH from this 10 M stock NaOH. Provide answers to the boxed areas shown below.
Fill flask with dI H2O
0.1 M NaOH to 500 mL mark
In the space below, show calculation set up to determine the volume, in mL, of 10 M NaOH you would use to make 500 -mL of ~0.1 M NaOH. In an actual lab, this volume of concentrated NaOH would be placed in a 500 -mL flask, and dI H2O is added till the water level reaches the 500 -mL mark.
2. You have now prepared a 500-mL dilute solution of ~0.1 M NaOH. Because the NaOH concentration is only approximate, you need to standardize it against the primary standard potassium hydrogen phthalate, KHP, a monoprotic organic acid. A primary standard is a substance whose composition is known with high precision, which is stable against typical laboratory storage conditions and which reacts in a clean and consistent way with the reagent in question. The standard of choice in a titration experiment to standardize a NaOH solution is KHP. The titration reaction between KHP and NaOH is shown below:
KHP(aq) + NaOH(aq) → NaKP(aq) + H2O(aq)
Potassium hydrogen phthalate Sodium potassium phthalate
To standardize the dilute ~0.1 M NaOH solution, a titration will need to be performed. The procedure calls for calculating the mass of KHP that would react with 5.00 mL of 0.1 M NaOH. The precise concentration of the dilute NaOH solution will be determined based on the volume of NaOH used in the titration and the exact mass of KHP. Provide an answer to the question in the boxed area below.
In the space below, show calculation set up to determine the mass of KHP that would react with 5.00 mL of 0.1 M NaOH. The molar mass of KHP is 204.44 g/mol. The titration reaction is shown on page 2. (Note: Because a volumetric pipet is used, which is precise to 2 decimal places, the 5-mL volume is recorded as 5.00)
3. Titration Apparatus Set Up
► In a manual titration procedure that you may have performed in the past, the titration apparatus would consist of a buret holding the ~0.1 M NaOH solution, and the reaction flask would contain the measured KHP, which is dissolved in certain arbitrary volume of water + a few drops of a color indicator, as shown in the cartoon at right. A small volume of base is added at regular interval, and the corresponding pH change recorded manually. The volumes of base added are read off the buret’s graduation markings. The procedure is performed till the titration endpoint is reached, as signaled by a change in the color indicator. The total volume of base dispensed at the equivalent point is then recorded. A few more additions of base are made beyond the endpoint, and a titration plot of pH versus volume base added can be manually constructed on graph paper.
A titration apparatus with a buret holding the titrant.
► In this lab, the procedure calls for using an automated electronic titration apparatus that uses a drop counter to measure the volume of base dispensed and Loggerpro Vernier software to plot the titration curve and to monitor the titration in real time (see drawing of apparatus set up on page 5). In place of a buret, a large 60-mL syringe is used as the reservoir of the base. The volume flow of base is adjusted manually via the stopcocks located at the tip of the syringe.
The drop counter has a slit opening through which the base is added one drop at a time. As the drop goes through the slit, it interrupts a light beam and the event is registered by the computer. Because the drop counter has been previously calibrated, the total number of drops can be translated into milliliter volume.
Slit opening where drops of NaOH solution pass
Drop counter through.
Titration reaction in which dissolved KHP and NaOH react with one another.
Enlarged view of drop counter
itration is monitored in real time
new data points
of pH vs
plotted by computer software.
► Please watch the YouTube video (5 min 43 sec) at the link below to give you a sense of how this type of titration apparatus is set up and conducted: https://www.youtube.com/watch?v=z0c3HCVF79k
Once the titration curve is generated, Vernier computer software is used to analyze the curve. For example, the inflection point is the point at which the slope of the rapid rise in pH starts to decline. It marks the equivalence point of the titration. The corresponding volume of base added to reach the equivalence point can be precisely obtained from the computer readout.
Volume of base added at the equivalence point.
Volume of NaOH added (mL)
4. The standardization titration of the 0.1 M NaOH is carried out in triplicate, and the average result of the three titrations will be taken as the accepted concentration of the dilute NaOH solution.
Set up three titration reactions for KHP and NaOH:
▪ Record the exact mass of KHP measured on the analytical balance
▪ Perform three trials of KHP titration with your prepared dilute NaOH solution
0.102 g KHP
(measured mass of KHP) 30 mL dI H2O
100 mL beaker
Trial 1 Trial 2 Trial 3
▪ For each trial, calculate the molarity of the dilute NaOH solution based on the mass of KHP and the volume of base used to reach the equivalence point. Crucial data for the standardization titration have been entered for you in the data table on page 7. Please perform the required calculations to fill the rest
of the table. Below the table, provide a sample calculation using data in trial 1 to show how you obtain the results. For full credit, all calculation results must be rounded off to correct sig figs and with proper units.
Data Table for Part A
Mass of KHP (g)
Volume of NaOH at equivalence point (mL)
Molarity of NaOH (M)
Average molarity (M) of NaOH solution
Standard deviation (must have same number of decimals as ave. molarity)
Part B: Determining the Ionization Constant of the Unknown Acid
Now that you have a standardized base, you are ready to use it to titrate your unknown acid. You will use the average molarity value of the standardized NaOH obtained in Part A to perform calculations in this section of the experiment. The titration experiment is performed similarly as in Part A, with the same set up that makes use of a drop counter and Loggerpro Vernier software for data analysis. However, unlike in Part A where you perform the titration against a given mass of KHP, in Part B you will titrate 5.00 mL of an unknown acid against your standardized base solution.
As in most titration experiments, the titration with the unknown acid is carried out in triplicate, and the three results are averaged to obtain the final consented value. The experimental preparation for the titration of the unknown acid is shown on the next page.
1. The titration reaction between the unknown acid, HA, and NaOH is as follows:
HA(aq) + NaOH(aq) → NaA(aq) + H2O(aq)
2. Set up three titration reactions:
▪ Perform three trials of HA titration with your standardized NaOH solution
5.00 mL unknown
acid 30 mL dI H2O
100 mL beaker
Trial 1 Trial 2 Trial 3
At the top of page 9 is the titration curve result of trial 1 titration experiment. The volume of your unknown acid is 5.00 mL. The 30-mL volume of dI H2O used is arbitrary and does not influence the end result because the amount of base added to neutralize the acid depends on the mol amount of acid in the 5.00 mL sample, not on the concentration of the acid in the titration reaction. Hence, the initial concentration of the unknown acid is calculated based on its 5.00 mL volume.
In an actual experiment that consists of 3 trials, you would have 3 titration curves to analyze.
However, because I have not performed the actual experiment to generate the triplicate data, I will present you with just one titration curve to analyze, and let’s call it trial 1 data. Therefore, you will need to do analysis only on one data set and provide the results in the data table shown on page 10. While three data sets are desirable for a more reliable result, nevertheless having only one data set provides you with some practice and gives you a sense of how a titration curve is analyzed. Unfortunately, a major objective of this lab that cannot be met is to let you learn how to set up the drop counter titration apparatus and to use the Vernier software to analyze the titration data. Obviously, the hands-on experience of this lab cannot be duplicated virtually at this point !!
Titration data for Part B
Below is a titration curve that would be obtained for trial 1 of this experiment.
Vol at eq. pt.
0.00 I I I I I I I I I I I I I I I I I
0 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00
Vol of NaOH (mL)
▪ Your tasks: From the titration curve shown above, obtain the necessary data as precisely as you can (with proper sig. figs) to determine the following:
- the volume of NaOH used to reach the equivalence point.
- the ionization constant (Ka) of the unknown acid.
- the molarity, in M, of the unknown acid (HA) solution. Note that the molarity of the standardized base used in this titration is the average molarity obtained in Part A.
- perform the necessary calculations and tabulate your data in the data table on page 10. For full credit, where appropriate all numerical data should have correct sig. figs and proper units.
Data Table for Part B
Volume of NaOH at
equivalence point (mL)
Molarity of HA solution
Discussion and Calculations
In the boxed area below, provide calculation set ups to determine the Ka and the molarity of the unknown acid sample and briefly explain the rational of your method.
Question 1: (1 pt) Of the following solutions, which has the greatest buffering capacity ? (Circle the correct answer.)
- 0.821 M HF and 0.217 M NaF
- 0.821 M HF and 0.909 M NaF
- 0.100 M HF and 0.217 M NaF
- 0.121 M FH and 0.667 M NaF
- They are all buffer solutions and would all have the same capacity.
Question 2: (2.5 pts) Calculate the pH of a solution prepared by mixing 45 mL of 0.183 M KOH and 65 mL of 0.145 M HCl. For credit, show work set up leading to the correct answer in the boxed area below. Set the answer in correct sig figs.
Question 3: A sample of a 0.100 M H2A (a diprotic acid) solution was titrated against a standardized solution of 0.100 M NaOH. The titration curve of pH vs volume of base added was plotted, and the result is shown below.
- (0.5 pt) Is it possible to determine the volume of the acid sample from the titration data ? If no, then indicate so. If yes, than what is the volume (in mL) of the H2A sample being titrated ?
- (0.5 pt) At approximately what pH is the concentration of HA− equal to the concentration of A2− ?
- (0.5 pt) What is the approximate [H+] at the second equivalence point ? For credit, show work set up leading to the correct answer.
(Post-lab question 3 continued)
- (0.8 pt) What is the exact concentration of A−2 ion at the 2nd equivalence point ? For credit, show work set up leading to the correct answer. For full credit, write the answer in correct sig. figs and with proper units.
- (0.8 pt) Write the balanced equation for the reaction that occurs at the 2nd equivalence point.
- (1.6 pt) Estimate the Ka1 and Ka2 for H2A. Show all work set up for credit. Answer without calculation set up will receive NO CREDIT. For full credit, write the answers in correct sig.
figs and, where applicable, with proper units.