A. Preparation of the 0.040 M NaOH
- Use a 10 mL graduated cylinder to measure out ~ 3.2 mL of 2.5 M NaOH.
- Add this to a 200 mL volumetric flask.
- Fill the flask to the line with deionized water.
- Cork the flask and carefully invert it several times to make sure the solution is homogenous.
B. Standardization of the 0.040 NaOH Solution
- NaOH is hydroscopic and absorb water from the air causing the concentration of any sodium hydroxide solution to change over time. It is necessary to calculate the exact concentration of NaOH by titrating with a primary standard.
- Label two 50 mL flasks for Trial 1 and Trial 2. Measure between 0.1 and 0.13 g of KHP (HKC8H4O4).
- Record the exact mass to 0.001 g in your data section.
- Dissolve the KHP by adding ~ 15 mL water to the flask. You may need to swirl for several minutes to dissolve.
- If there is KHP on the sides of the flask use a water bottle to help dissolve the solid.
- Add 3-4 drops of indicator to each flask.
- Check your buret to make sure it will not leak by adding deionized water to the top and allowing it to run out as shown by your instructor.
- Carefully add ~ 50 mL of the ~ 0.040 M NaOH solution to a buret. Do not waste time getting it to the 0.0 mark. Instead record the exact volume (to two decimal spaces) in your data section.
- Titrate the sample slowly until a pale pink color remains for at least 3 seconds. If you exceed the endpoint the color will be a dark pink and you will have to do a third trial.
- Obtain instructor approval on the color of the solution before moving on.
- Record the final volume of NaOH in the data section. You do not need to add more NaOH to the buret between trials. Use the final volume from the first trial as the initial volume for the second trial.
- Use the volume of NaOH and grams of KHP to calculate the molarity of the NaOH solution. (If you need help, see the equations in your prelab).
- Record the concentration of your standardized NaOH solution in your manual. You will use this molarity in Part C and Part D of today’s experiment.
C. Titration of a Carbonated Beverage
- Obtain ~ 20 mL of soda. Record the brand of soda in your data table.
- If necessary heat the soda for 10 minutes to approximately 80 degrees C to remove carbon dioxide. This step is not necessary if your instructor has already performed this step for you.
- Use a pipette or stirring rod to collect a drop of the cooled soda.
- Touch the drop to a piece of pH paper. Record the pH of the beverage in your data section.
- Use the pH to evaluate what the concentration of H+ in the soda should be. You will confirm this with your standardized NaOH.
- Clean two 50 mL flasks. Label them Trial 1 and Trial 2.
- With a clean 10 mL graduated cylinder measure between 2.8 and 3.2 mL of soda.
- Add the sample to your 50 mL flask. Be sure to record the exact volume of soda used for each trial.
- Add 3-4 drops of indicator.
- Add ~ 20 mL of deionized water to make the titration easier. (If the volume of the analyte is too small, it will be difficult to distinguish the color change).
- Fill the buret with ~ 50 mL standardized NaOH. Record the initial volume in your data table.
- Titrate the soda to a light pink color. Obtain instructor approval before moving on.
- Record the final volume of the standardized NaOH in your data table.
- Using the balanced equation calculate the concentration of acid in your soda sample.
D. Titration of Citric Acid
- Obtain a clean 50 mL flasks.
- Weigh out 0.05 g citric acid.
- Dissolve the sample in ~ 20 mL of water. Add 3-4 drops of indicator.
- Fill the buret with ~ 50 mL of standardized NaOH. Record the initial volume in your data table.
- Titrate the citric acid to a light pink color. The color should be maintained for at least 30 seconds. If it fades to colorless, add another drop of sodium hydroxide to the sample.
- Record the final volume of NaOH in your data table.
- If time allows, do another trial.
*Note– this pre-lab must be finished before you come to lab.
1. Calculate the molar mass of citric acid C6H8O7.
2. Calculate the volume of 2.5 M NaOH needed to make 200 mL of 0.04 M NaOH.
3. The balanced equation for KHP and NaOH is given in the introduction to this lab. Calculate how much KHP is needed to react ~20 mL of 0.040 M NaOH.
4. The pH of flat sprite is usually 4.16. Calculate the H+ concentration in the beverage. (Remember that pH = -log[H+]).
5. Citric acid, the main acidic component of Sprite, is triprotic. What is the concentration of citric acid in Sprite? (Hint: Use the molarity of H+ and the mol : mol ratio of citric acid to acidic protons to calculate it’s concentration.
Experimental Data and Results
A. Preparation of NaOH
|1. Concentration of Stock Solution|
|2. Volume of Stock Solution Used|
|3. Volume After Dilution||200.0 mL|
|4. Final Concentration (approximate)|
B. Standardization of NaOH
Show your work.
|Trial 1||Trial 2||Trial 3 (only necessary if instructor does not give approval for one of the previous trials).|
|1. Mass of KHP|
|2. Mol KHP|
|3. Mol NaOH|
|4. Initial Volume NaOH|
|5. Final Volume NaOH|
|6. Volume NaOH Used|
|7. Instructor Approval of Endpoint|
|8. Molarity of NaOH|
Average Concentration of NaOH ______________________________________
*This is the concentration of NaOH you use in C.
C. Titration of Carbonated Beverage.
Show your work.
|Trial 1||Trial 2|
|1. Volume of Soda Used|
|2. Standardized Concentration of NaOH|
|3. Initial Volume NaOH|
|4. Final Volume NaOH|
|5. Volume NaOH Used|
|6. Instructor Approval of Endpoint|
|7. Mol NaOH|
|8. Mol to Mol Ratio of NaOH to Citric Acid||3 NaOH : 1 Acid||3 NaOH : 1 Acid|
|9. Mol Citric Acid|
|10. Concentration of Citric Acid in the Soda|
Did the titration show that citric acid is triprotic? Why or why not?
1. Why isn’t it necessary to measure the volume of water added to the analyte in a titration?
2. If the stock solution of NaOH was standardized yesterday morning, is it necessary to standardize the solution today? Why or why not?
3. How could we have modified today’s experiment to perform analysis on a dark soda?
4. Using the concentration of citric acid in your sample of Sprite (see #10 in the table from Part C: Titration of Carbonated Beverage) calculate the pH of your Sprite sample.
5. Does the concentration of citric acid in your Sprite sample agree with the theoretical concentration of citric acid in Sprite (see your answer for prelab question 5)? Explain.