In your lab notebook, start with a fresh right-hand page, and, in addition to the names, date and lab title, complete the purpose of lab 3. Don’t forget to update the table of contents at the front of your lab notebook. You may cut and paste the “Skills” section, but write the chemical equation for this lab, augment the chemicals and equipment lists as needed, draw a sketch of the Buchner funnel filtration step, and write the Ksp equation needed for this lab.
The “Procedure” section may be printed and pasted, but it contains some choices in parentheses; circle the appropriate ones. The “Data” section will contain tables that are print-outs; you may edit them to show the relevant sections. The “Analysis” section will contain a couple graphs; the rest of the lab write-up, including the abstract, is straightforward.
A copy of the lab 3 notebook pages and the abstract is due Wednesday, May 2 in class.
The point of this lab is how you can measure and calculate Ksp. One way to do that would be to make a saturated solution of the slightly soluble salt, then boil off the water and weigh what remains. This is not practical for a couple of reasons. The method described below is more circuitous but should yield a good result.
Chemistry 163
Lab 3: Calculation of the solubility product of an organic salt
Purpose: To determine the value of the solubility product of an organic salt, using a ______.
Skills: • Use of a drop counter for precision volume measurement
• Use of a digital pH meter for quantitative determination of the acidity/alkalinity of a solution
• Use of a Buchner funnel for filtration
Chemical equation:
Chemicals needed: • Potassium hydrogen tartarate (KHC4H4O6 – 188.18 g/mol)
• Roughly 0.04 M NaOH
• distilled water (dH2O)
Equipment needed: • Labquest data logger and digital pH meter
• Drop counter with stirrer, with ringstand and clamps
• 150 mL and 250 mL beakers
• Buchner funnel with sidearm (filtration) flask
Special equations needed: Ksp =
Procedure:
1. Set up and calibrate the drop-counting apparatus, as stated in the “Using a Drop Counter with pH probe”, as done in previous labs.
2. Set up the Buchner funnel on the sidearm (filtration) flask, and attach the rubber hose to connect the sidearm to the aspirator on the sink. Make sure you are drawing a vacuum through the funnel. Remember to break the vacuum (by uncoupling the hose/ by shutting off the water).
3. Weigh about 2 grams (record the exact amount) of finely powdered potassium hydrogen tartarate into a clean 250 mL beaker. Add about 150 mL of distilled water and stir well. Given that the solubility of potassium hydrogen tartarate is 1.0g/162mL at 25°C and 1.0g/16mL at 100°C, you (will/will not) see excess powder on the bottom of the beaker.
4. Pour the contents of the beaker into the Buchner funnel and perform a vacuum filtration. (Do/do not) rinse the beaker out into the funnel.
5. Record the temperature of the filtrate, then pipet 25 mL of the filtrate (using the appropriate volumetric pipet) into a clean, dry 150 mL beaker. Place the beaker under the drop counter and make sure the pH meter and magnetic stirrer are set up. If necessary, add 25 mL of distilled water to the beaker.
6. Record the exact concentration of the sodium hydroxide (units, too) in the data section of your notebook.
7. Fill the reservoir of the drop counter with the sodium hydroxide solution, and titrate the potassium hydrogen tartarate solution. Remember, you will need to go past the equivalence point, since you (do/do not) know the pKa of the hydrogen tartarate ion.
8. After obtaining a satisfactory equivalence point, repeat the experiment with a fresh batch of potassium hydrogen tartarate powder.
Data
A table with NaOH concentration, filtrate temperature and other relevant non-titration data.
Depending on the number of data points generated by the drop counter, print out, edit and tape into your notebook the equivalence point portion of these data. Don’t forget column headings and units.
Analysis
A titled, labeled graph of each titration run (may be overlaid on the same graph) indicating equivalence point volume.
A series of calculations for each run showing the transformation of volume of NaOH used, to moles of OH–, to moles of HC4H4O6–, to Ksp. This set of calculations should be organized logically and the final Ksp clearly marked. Do pay attention to sig figs.
Group results
Enter your initials and each Ksp in separate columns on the spreadsheet on the computer at the front of the classroom. Calculate, for each column, the class average and standard deviation.
Research
Find a reputable source (accepted value) of the Ksp of potassium hydrogen tartarate. Report that value, and the full citation (use a standard scientific citation format) of the source.
Questions
1. What are you assuming about the filtrate and potassium hydrogen tartarate?
2. Suppose you did add 25 mL of distilled water to the filtrate prior to the titration, then carried out the calculation without taking the dilution into account. Would this affect your Ksp. If so, would the solubility product be larger or smaller? Show this in a semi-quantitative way. If not, explain why the dilution does not affect Ksp.
3. Estimate the pKa of the hydrogen tartarate ion from your graph.
4. Phenolphthalein is an indicator that changes color from clear to pink at around pH 9. Would this have been an appropriate indicator to use if we didn’t have the drop counter? Explain.
Conclusion
Calculate and report a percent error between your team’s average Ksp value and the accepted value you found. Calculate and report a percent error between the class’s average Ksp value and the accepted value you found. Comment on whether the class as a whole suffered a random or a systematic error (or both) based on the percent error and the standard deviation.
For either type of error, suggest one source of error and how that helps explain the type of error you saw.
Comment on whether this method could yield a Ksp value with more than two sig figs. What measurement (or constant?) limits the precision of this value? Could an equipment or technique change yield more sig figs? If so, suggest how.
Abstract
In 125 words or less, and using the format in previous abstracts, summarize the main result of this lab (namely, the Ksp values obtained by your team). An assessment of how “good” these values are is in order, both in terms of precision and accuracy. Finally, comment on the significant source of error your team encountered, and a suggestion to fix that error.