This is a Journal article published in Journal of Institute of Science and Technology, tribhuvan University Nepal in 2005. Please cite this paper as: Giri B. and Pradhananga R., Location of …Full description
A discussion regarding potentiometric titrations with the use of ph meterFull description
Deskripsi lengkap
Descripción completa
Full description
Full description
Here is somthing for the lady's Building a better butt and body for Female's By Brett Contreras AKA (the glute guy)
Road designFull description
Here is somthing for the lady's Building a better butt and body for Female's By Brett Contreras AKA (the glute guy)
Here is somthing for the lady's Building a better butt and body for Female's By Brett Contreras AKA (the glute guy)
hi
highway curves
Full description
Descripción: Here is somthing for the lady's Building a better butt and body for Female's By Brett Contreras AKA (the glute guy)
Cost CurvesFull description
oil & gas
Chemistry 153 Lab
ACID/BASE TITRATIONS & TITRATION CURVES Acid-base titration is a useful technique for determining the concentration o f an acidic or basic solution. As you will see today, titration of a weak acid or base can also be used to determine the K a or K b for the acid/base. You have already been introduced to titration technique in previous labs. Today you will use a pH meter connected to a computer to generate a titration curve. A titration curve shows h ow the pH of a solution changes as acid or base is added. The shape of a titration curve is influenced by both the concentration and nature of the acid and base. You will see three different titration curves in today's lab. A typical strong acid-strong base titration curve is shown below. In previous titration experiments you have determined the endpoint end point (or equivalence point) of the titration by observing the color change of an indicator that is added to the solution. When using a pH meter to monitor the titration, the endpoint is determined b y the inflection point of the curve. This provides a more accurate determination of the endpoint. The inflection point is the steepest part of the curve, and is also the point on the curve where the curvature changes. [In calculus terms, this is the point where the second derivative equals zero.]. Please review your text for a further discussion of titration curves, particularly the discussion of titrations involving weak acids and bases.
In this lab you will work with a partner p artner to complete two titrations. First you will titrate a weak acid (KHP) with a ~0.1 M sodium hydroxide solution. From the results of this titration you will be able to calculate the exact concentration of the NaOH. (In other words, you will standardize the NaOH. Do you remember why this is necessary?) Next you will titrate an unknown polyprotic acid. From your titration data, you you will be able to calculate the acid ionization constants and the molecular weight of the unknown acid. A pH meter will be used to measure the pH as each reaction proceeds. So that you can monitor each titration in “real time”, computers will be available so that data may be entered and plotted as the titration proceeds.
Edmonds Community College Page 1 of 5
Chemistry 153 Lab
General Directions The following are general directions for each titration. Sp ecific details for each of the titrations follows. 1. Set up a titration apparatus as shown below. Position the pH electrode in the solution and adjust its position so that it is not struck by the stirring bar.
2. Open the graphing program Graphical Analysis from the Vernier programs menu. Label and scale the axes appropriately. 3. Use the buffer solutions to calibrate the pH probe as directed by the instructor. 4. Set up a buret with the NaOH solution provided. Fill to the 0.00 mL line. 5. For each titration, add the specified amount of acid to the beaker. If necessary add water to dissolve. Add a few drops of phenolphthalein to the acid. 6. Record the initial pH of the acid solution. This is the pH with no NaOH added. Record the initial volume (0.00 mL) of NaOH in the buret. Data should be recorded both in your lab notebook, and in the Graphical Analysis program. As you continue to enter data, the titration curve will be displayed on the screen. 7. Add NaOH a few drops at a time until you produce an approximate 0.2 pH unit change. Record the exact volume of NaOH added and the exact pH. Remember that burets should be read to the hundredths place. Edmonds Community College Page 2 of 5
Chemistry 153 Lab
8. Continue adding increments of NaOH to produce increases of about 0.2 pH units. Do not worry if you “overshoot” a change of 0.2 pH units, but be sure to record the exact volumes and pH values. Note: When the pH begins to change rapidly you will need to add base in much smaller increments in order to prod uce a change of only 0.2 units. 9. Monitor the graph of pH vs. volume added. After the pH rises steeply and b egins leveling off, you may end the titration. Remember that polyprotic acids will produce multiple endpoints, so take care that you do not end the titration too early. 10. Add a column to your Graphical Analysis data table to calculate the second derivative of the data. Save the file to your ACS account or to a removable drive. Do not save to the hard drive or your data will be lost.
Specific Directions for each titration: A. Titration of a known monoprotic weak acid (KHP) You will be given a vial containing KHP. Dissolve the entire sample in about 40 mL of water and determine the exact mass by weighing the vial before and after. Begin titrating with NaOH. Refer to your prelab for an estimate of the expected endpoint and add base in small increments as this point approaches. Continue titrating to be yond the endpoint.
B. Titration of an unknown polyprotic acid
You will be given a vial containing an unknown acid. Record the number of your unknown in your notebook. Dissolve the entire sample in about 40 mL of water and determine the exact mass by weighing the vial before and after. Titrate to beyond the final endpoint with NaOH, taking care to add base in small increments near each endpoint.
Edmonds Community College Page 3 of 5
Chemistry 153 Lab
Calculations Turn in your titration curves for each titration. 1. Determine the equivalence point for each titration and indicate the equivalence point(s) on each graph. 2. From the titration of KHP with NaOH, determine the exact concentration of the NaOH. 3. From the titration of KHP with NaOH, determine the Ka for KHP and compare with the accepted value. Do not expect exact agreement; look for order of magnitude agreement (same power of 10). 4. From the titration of NaOH with the unknown po lyprotic acid, determine the molar mass of the acid. 5. From the titration of NaOH with the unknown po lyprotic acid, calculate the ionization constants (Ka values) for the acid. 6. Consult the list of weak acids (posted on Blackboa rd) and by comparing your molar mass and Ka values, suggest an identity for the acid.
Questions 1. Why are the titration curves so steep near the endpoint?
2. Use the Ka value you calculated for your polyprotic acid to determine the pH of a 0.50 M solution of this acid.
Edmonds Community College Page 4 of 5
Chemistry 153 Lab
Name_______________________________
PRE-LAB: 1. Sketch the titration curve that should be obtained when a monoprotic weak acid is titrated with sodium hydroxide. Explain how and why this differs from a strong acidstrong base titration curve.
2. Estimate the expected endpoint of the titration when 0.30 g of KHP is titrated with 0.10 M NaOH.
3. Sketch the titration curve that should be obtained when a diprotic acid is titrated with sodium hydroxide.
4.
If the titration of 0.156 g of an unknown monoprotic acid requires 22.43 mL of 0.1027 M NaOH, what is the molecular weight of this unknown acid?
