Measurement of pH and Determination of Acid Dissociation Constant of a Weak Acid

Experiment 17

VIRTUAL LAB: VERSION HTML5 VIDEO WALKTHROUGH

HTML5 based version of the virtual lab. This beta release does not require the Java Plug-in and should run in most browsers. Please read the information below to help you get started.

Windows and Macintosh users: Please use FireFox or Chrome web browser.

The simulation will run on most laptops, desktops and touch-enabled devices such as tablets, iPads or phones.

If you have any technical trouble with the simulation initially loading on your page, often refreshing the page or restarting your browser will solve most issues. You can also email us for additional assistance.

To help you get started, please view a short video which introduces the virtual lab.

http://www.chemcollective.org/chem/common/vlab_walkthrouh_html5.phpPrelab Questions:

Define an acid and a base according to the Bronsted-Lowry theory.

Acids refers to proton donors while base refers to the acceptors of protons .

a. What is the concentration of H+ in 0.030M HNO3?

=0.03 moles in 1000 liters of the acid

What is the pH of the solution?

= -log (0.03)

=1.52

What is the OH- concentration in the solution?

= antilog (14-1.52)

=3.02 * 10 -12 moles

What is the pOH of the solution?

=14-1.52 = 12.48

Complete the following:

[H+] [OH-] pH pOH Acidity

1.8 x 10-6 M 6.3*10-9M 5.8 8.2 acid

2,754*10-5 M 3.6 x 10-10 M 4.56 9.44 Acid

7.07*10-9 M 1.412*10-6 M 8.15 5.85 base

5.01*10-9 1.995*10-6 M 8.3 5.70 base

Part I.

Virtual Laboratory Prelab Exercises for an Acid Base Titration

(Adapted from University of British Columbia)

Standardization of NaOH with a KHP solution: Acid Base Titration Info

http://chemcollective.org/activities/info/101Use the Virtual Laboratory to standardize an unknown NaOH solution (approximately 0.2M) to four significant figures via titration with 25.00 mL of a KHP standard solution.

This homework uses the virtual lab. Using a computer that is running Microsoft windows or Macintosh OS 10.1 or higher, go to http://ir.chem.cmu.edu and click on “Virtual Lab” in the upper left-hand corner. You can then either,

a) Run the lab as a Java Applet in a web browser by clicking on “Run the applet >>”.

b) Download and install the lab on your computer, by clicking on “download” at the bottom of the page.

To load the assignment, select “Load Homework…” from the “File” menu, and select

“Acids and Bases : Prelab Exercises, Acid Base Titration”.

Virtual Laboratory software offers interactive, simulation-based exercises that can replace paper-and-pencil Preparatory exercises in the Chem 112 laboratory.

In this pilot project you are asked to solve 4 problems using the Virtual Lab. The Virtual Lab homework will be marked out of 5 marks and 4 bonus marks will be offered for the participation in the project.

The Virtual lab homework answers should be submitted in your lab report. Two of the homework problems (#3 and #4) involve design of experiments.

Experiment #17: Homework for use with The IrYdium Project Virtual Lab

Use the Virtual Laboratory to standardize the unknown NaOH solution (approximately 0.2M) to four significant figures via titration with 25.00 mL of a KHP standard solution. What must be the minimum concentration of the KHP solution to yield the required accuracy in the NaOH results? Note that the buret should be read to 2 decimal places.

This problem should be done with the Transfer Bar toggled to Scientific Figures Transfer. In this mode, the Transfer Bar below the workbench will accept only values that reflect the accuracy of the glassware being used. For example, when you transfer a solution from a 25 mL pipet, type 25.00. When you attempt to pour a solution directly from an Erlenmeyer flask, type 25 and you will be able to transfer 25+/- 5 mL. The Solution Info window on the right will show only an approximate volume in the recipient flask. All volumes used in calculations must be read directly in the detailed view of the glassware.

Describe what glassware you had to use to obtain the required accuracy.

Tabulate your titration data (initial and final volumes).

Using the Virtual Laboratory, determine the maximum concentration of 25.00 mL of a KHP solution that can be titrated with 0.2M NaOH without having to refill the 50.00 mL buret during one trial. Again, read the buret to 2 decimal places and use “Scientific Figures Transfer mode”.

Using the Virtual Laboratory, design and perform an experiment to determine the concentration of the unknown HCl solution to four significant figures. The concentrations of the HCl, KHP, and NaOH are ~ 0.1M, 0.09000M, and ~ 1M, respectively. Use the “Realistic Mode” for this problem. In this mode, you cannot type in the amount to be transferred with the pipet, but rather you must visually withdraw liquid until the level matches the calibration line on the pipet.

Please outline your procedure and list the glassware used in each step. If in any step you diluted the reagents provided, what glassware did you use, and why?

Repeat the entire experiment in the Significant Figures Transfer Mode to check if your results from the Realistic Mode are correct.

4.Using the Virtual Laboratory, design an experiment to determine the concentration of the unknown HCl solution to four significant figures. The concentrations of the HCl, KHP, and NaOH are ~ 1M, 0.09000M, and ~ 0.1M, respectively. Use the “Realistic Mode” for this problem as per question 3. Again, please repeat the entire experiment in the Significant Figures Transfer Mode to check if your results from the Realistic Mode are correct. Also note that the HCl and NaOH concentrations are different from question 3.

Please outline your procedure and list the glassware used in each step. If in any step you diluted the reagents provided, what glassware did you use, and why?

Please Note:

The values in the Solution Info panel represent the actual volumes in the highlighted containers, their pH and concentrations of the species in solutions. These values should serve as a guide only. Do not use the Solution Info volumes/concentrations in any calculations, since you cannot actually measure them in the lab and they do not reflect the correct number of significant figures. Use only the values you can read off the pipet, buret, and graduated cylinder.

Part II.

http://chemcollective.org/activities/autograded/150Unknown Acid Problem

Your company has just purchased a warehouse containing an old chemistry laboratory, and the first order of business is to take inventory of the chemicals in the stockroom so that you can properly dispose of them according to current EPA regulations. Unfortunately, the previous chemist did not label the bottles very thoroughly and left two bottles marked simply “acid” with no further information.

Fortunately, you find an old notebook in the lab with a table of the acids used by the previous chemist. (See table below).

You decide that you can collect some experimental data which will enable you to both (1) identify the unknown acids by comparison with the lab notebook information, and (2) determine their concentrations. Since the disposal of unidentified chemicals is more expensive than that of known compounds, a few hours of lab work can translate into considerable savings for your company.

Acids pKa

crotonic 4.69

methylmalonic 3.07

diphenylacetic 3.94

picric strong

alloxanic 6.64

methyl-m-aminobenzoic 5.10

You can submit your answers up to three times. If all three answers are incorrect, you will be given the correct answer and asked to reload the page and try a new problem. Please be aware that reloading the page will result in having to start a new problem.

Please enter your answer in the form below, using 3 significant figures for the concentration

The unknown acid in the first bottle is:

The concentration of the unknown acid is: M.

Part III.Unknown Acid and Base Problem

Determining the pKa of a weak acid and weak base

In this exercise, students graph the titration curve of an unknown acid and base to determine their pKa’s and concentrations.

http://chemcollective.org/activities/info/103

a) Run the lab as a Java Applet in a web browser by clicking on “Run the applet >>”.

b) Download and install the lab on your computer, by clicking on “download” at the bottom of the page.

To load the assignment, select “Load Homework…” from the “File” menu, and select “Acids and Bases : Unknown Acid and Base Problem ”.

(5 pts) The “Homework Solutions” cabinet contains a solution labeled “Unknown Acid,” which is a weak mono-protic acid with an unknown Ka and with an unknown concentration. Your job is to determine the concentration and Ka to two significant figures. Please report your results and explain your procedure. The concentration of the of the unknown acid through titration with the KHP sample while slowly adding Sodium Hydroxide solution to the mixture. The data obtained is recorded for the end points of the reaction.

2) (5pts) The cabinet labeled “Homework Solutions” also contains a solution labeled “Unknown Base,” which is a base with an unknown Kb and with an unknown concentration. Your job is to determine the concentration and Kb to two significant figures. Please report your results and explain your procedure.

Part IV.

http://chemcollective.org/activities/autograded/143Determine the identity of an unknown solid weak acid

The virtual lab stockroom contains NaOH, water and an unknown solid weak acid. Perform experiments to identify the pKa and molar mass of the bottle of unknown solid acid. You may enter your answer in the form below the Virtual Lab.

NOTE: You can submit your answers up to three times. If all three answers are incorrect, you will be given the correct answer and asked to reload the page and try a new problem. Good Luck! Please be aware that reloading the page will result in having to start a new problem with a different unknown acid.

Please enter your answer in the form below.

The unknown solid acid has a molar mass of: grams/mole and a pKa of:

Part V.

VIRTUAL LAB: Determination of the pH Scale by the Method of Successive Dilutions

Instructions for Virtual Simulation Lab video walkthrough

To help you get started, please view a short video which introduces the virtual lab.

http://www.chemcollective.org/chem/common/vlab_walkthrouh_html5.phpSimulation Lab

http://chemcollective.org/vlab/100Determination of the pH Scale by the Method of Successive Dilutions

a) Run the lab as a Java Applet in a web browser by clicking on “Run the applet >>”.

b) Download and install the lab on your computer, by clicking on “download” at the bottom of the page.

To load the assignment, select “Load Homework…” from the “File” menu, and select

“Acids and Bases : Method of Successive Dilutions ”.

Objective:

It is fairly common knowledge that neutral water has a pH of 7, acids have a pH <7 and bases have a pH>7, but few people understand this in terms of the actual hydronium ion concentration. Our objective is to develop an understanding of logarithmic scales by developing a pH scale

Background:

The pH scale describes the hydronium ion concentration in aqueous systems

pH = -log[H3O+]

[H3O+] = 10-pH = 1/10pH

The Method of Successive Dilutions is an experimental technique for preparing a series of solutions of different concentrations from one volume of stock solution.

Let’s look at a series of half dilutions.

With the virtual lab fill 5 flasks with a constant amount of water (less than half the volume of the flask), for simplicity, we will use 20 ml, but any amount will do.

*Virtual Lab Tip* – Right Click on each flask and label it

Now add the same amount of stock 1M HCl to the first flask (20 mL) and note that the concentration has been diluted in half, [H3O+] = 0.500M or 1/2 (1/21) the original molarity.

From this flask transfer 20 mL to the second flask and note the it has been diluted in half again,[H3O+]= 0.250M or is one fourth (1/22) the concentration of the original stock solution.

Repeating this procedure with the remaining 3 flasks gives:

3rd dilution: [H3O+] = 0.12500 or 1/8 (1/23) the original stock molarity

4th dilution: [H3O+] = 0.06250 or 1/16 (1/24) the original stock molarity

5th dilution: [H3O+] = 0.03125 or 1/32 (1/25) the original stock solution.

Let’s look at this in more detail:

[H3O+] = 2-n = 1/2n

Where n is the number of successive dilutions and by using a dilution factor of one to two, you have come up with a log base 2 scale.

Question: Would changing the volume of the original stock solution and the incremental dilution volumes to a new constant value effect the successive concentrations. Say by starting with 10 mL and transferring 10 mL increments? If you say yes, repeat the above with 10 mL increments and explain.

Assignment:

Make (and label) 7 Solutions by choosing the appropriate dilution factor so that each solution has a pH of 1-7.

1. What is the dilution factor you need and describe how you ran two sets of dilutions. Be sure to state how much water you initially placed in each flask and how much solution you successively transferred. = 1/27

2. Make a table with 3 columns; the number of the dilution, the concentration of the resulting solution and the pH of the resulting solution.

3. Write an algebraic equation based on your dilution factor, show that n = pH, and give the value of n for the original stock solution

4. Make a series of plots of [H3O+] (y-axis) vs. pH – number of dilutions (x-axis). Do these on three different sheets of paper with different [H3O+] scales.

Have the first one cover [H3O+] from 10-3 to 10-1 M and pH 3-1

The second cover [H3O+] from 10-5 to 10-3 and pH 5-3

The third cover [H3O+] from 10-7 to 10-5 and pH 7-5.

Arrange these sheets in order of smallest H3O+ concentration to largest (bottom of the y-axis being smallest, top being largest). Note how the magnitude of each scale has changed as you transcend from one sheet to the next.

Now take a 4th sheet of graph paper and try and squeeze all the data into one graph with a scale going from 10-7 to 10-1 (bottom to top) and pH 7-1 across.

5. Make a solution of pH of 8. Explain why further diluting will not work. This is because the pH of the acid has extended beyond the PH and has achieved neutralization.

6. (Optional) What dilution factor would result in an exponential decay e-n? The dilution factor, 1/10 would result in an exponential decay e-n.

Part VI.

Virtual Lab: Titration

http://www.mrpalermo.com/virtual-lab-titration.htmlPrint this LabIntroductionIn chemistry laboratory, it is sometimes necessary to experimentally determine the concentration of an unknown acid or base solution. A procedure for making this kind of determination is called an acid-base titration. In this laboratory process, a solution of known concentration, called the standard solution, is carefully added to a solution of unknown concentration until the mixture becomes neutral. The neutral point of the solution is recognized by an indicator’s color change. If the unknown solution is acidic, then the standard solution will be basic. The opposite would be true if the unknown solution was basic.We know that the mixing of equal amounts of acid and base ions will create neutral water. At the molecular level, this reaction can be illustrated with the following equation. H+ + OH- –> H2O (acid) (base) This equation states that one mole of hydrogen ions (acid) will neutralize one mole of hydroxide ions (base). Since we can exactly measure the moles of the standard solution, we can assume that the moles of the solution of unknown concentration will be the same at the neutral point. This is called the end-point of the titration. Using the equation MaVa = MbVb, we can use the experimental data from the titration to find the unknown concentration.Objective:

In this experiment the unknown solution will be HCl (aq) and the standard solution will be the base sodium hydroxide. You will know the concentration of the base and the volume of the acid and base used. With this information you can use the titration formula to calculate the concentration of

the acid. The diagram below shows the set up.

Pre Lab: Define the following words: Titration- _analytical method of determination of concentration of unknown through reacting with known concentration of regent________________________________________________________

Endpoint- ___this is the point when no reaction occurs________________________________________________________

Neutralization- _____________This is when an acid and base react to from a salt and water__________________________________________write the neutralization reaction for HCl reacting with NaOH.

NaOH + HCl → H2O and NaCl

What is the pH of the solution at the end point of the titration?

The pH of the solution is expected to be around 7 for the end of the titration.

Procedure:

The flask is filled with 10 mL of unknown concentration of HCl. (Click here)

Initial volume of acid in burette= 2mL

Final volume of acid in burette. = 12mL

Record the volume of acid on your data form

=12-2= 10 mL

Phenolphthalein Indicator is added to the flask. (click here)

Record on your data form the initial volume of base in the buret (click here)

= 2mL

You will now start adding base (0.25M NaOH) from the buret into the flask to neutralize the acid. The flask is gently swirled as the base is added. A pink color should appear as the base is added, but it will disappear as the flask is swirled.

The end-point of the titration will be reached when 1 drop of base makes the solution turn and stay pink.

(Click here).

Record the final volume of base from the buret that was needed to neutralize the acid. (click here)

= 16mL

Data:

Volume of acid=12-2= 10 mL

Volume of base =16-2 = 14 mL

Calculation: Determine the Molarity (concentration) of HCl using the data you collected and the titration formula. (The concentration of the NaOH used was 0.25M) Record your answer on your data form in your notebook.

Moles in NaoH= 0.25 moles in 1000 mL, 14= 14*0.25/1000= 3.5*10-3 moles

Moles of acid= moles of base, Molarity of acid= 3.5*10-3 moles in 10 mL, 1000= 1000 * 3.5*10-3/ 10 =0.35 M

Questions

State the purpose of this experiment.

To determine the concentration of HCl using the titration method

Describe the function of the phenolphthalein in this experiment. How would this experiment be different if you had forgotten to add the phenolphthalein

The phenolphthalein is essential for identifying the end point of the reaction without it would be difficult

State evidence from the lab that the endpoint was reached.

The was a noted color change in the mixture from colorless to pink

It takes 75ml of a 2.5M HCl solution to neutralize 55ml of a base of unknown concentration. Calculate the concentration of an unknown basic solution.

Rem according to the mole reaction 1mole of acid reacts with a mole of the base.

2.5 moles =1000ml, 75mL = 75 *2.5/1000= 0.01875 moles

Moles in unknown= 0.01875 in 55 ml; 1000mL =1000 *0.01875/55 = 0.3409 Moles

If the actual concentration of the HCl was 0.33M what is your percent error?

(0.3409 – 0.33/0.33)100 = 3.30%

Environmental studies usually involve an analysis of precipitation and its response to pollution. To quantify the degree of contamination in natural rain water or snow, titration is used. The process is quick and results are reliable. Since most titration processes do not require expensive or specialized equipment, the test can be performed often and in different areas with relatively little effort.

Post Lab Questions:

A 1000.0 mL sample of lake water is titrated using 0.100 mL of a 0.100 M base solution. What is the molarity of the acid in the lake water?

Moles in base = 0.1moles in 1000ml, 0.1 ml =0.1 *0.1/1000 = 1*10 -5

= rem moles acid = moles of base = 1*10 -5 moles in 1000 ml = 1*10 -5 * 1000/0.1 =

b. Based on the molarity of the acid calculated above, what is the pH of the lake?

Water?

= -log (1*10 -5) =5

Measurement of pH and Determination of Acid Dissociation Constant of a Weak Acid

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