Purpose: To determine the molarity of a KOH solution by performing a titration using 0.100 M HCl.
1. Obtain 0.100 M HCland? M KOH.
2. Prime each buret with about 2-3 mL of solution. Keep the proper solution in the proper buret!
3. Fill each buret with the proper solution, open the valve just long enough to let out any air bubble. Carefully read the initial amounts & record in the data table.
4. Place the Erlenmeyer flask under the HClburet, open the valve and allow 20-25 mL of the HCl to flow into the flask. Then close the valve.
5. Add 2 drops of the phenolphthalein indicator to the HCl in the flask.
6. Time to titrate! Place the Erlenmeyer flask under the KOH buret and open the valve to allow the KOH to flow into the flask while continuously swirling the flask. Observe the color changes occurring. Continue to add the KOH slowly while swirling the flask. When a faint pink color appears and persists for 5 seconds or more of swirling of the flask, you have reached your endpoint.
Note: It is important to realize that if you accidentally pass your endpoint, you can add more HCl to the flask which will cause the solution to become colorless again. You can then add the KOH slowly to the flask again and attempt once more to carefully reach the endpoint. This is known as “back-titrating.”
7. When you have reached the endpoint, recordboth of the HCl and KOH final buret readings in the table (again to the bottom of the meniscus and again to the hundredth of a mL).
8. Calculate the molarity of the KOH solution.
9. Pour the pink solution out of the flask (into the sink) and repeat the titration again-- no need to rinse the flask, only this time using 15-18 mL of the HCl solution. You should estimate how much KOH you will need for this titration based on the information that you gathered from the first titration.
10. Calculate the molarity of the KOH solution from the second trial. It should be very close to the molarity you got for the first trial. Then calculate your average. You will be graded on your accuracy, so do a careful job.
Data Table (Please copy this into your notebook):
Calculations (in your notebook): Calculate the molarity of KOH for each trial and the average KOH. Show all work and remember to include a balanced equation! Check with Ms. Lee when finished (you may have to do 3 trials.
Post Lab Questions/Homework(to be completed in your lab notebook):
- Consider if each of the following potential error sources would have caused your calculated value for ?M KOH to come out too high, have caused it to come out too low, or if it would have no effect at all on your value. Answer and explain Ityour reasoning.
- There was a little distilled water in the Erlenmeyer flask before you began the titration.
- There was a little distilled water in the HClburet and you forgot to rinse it out with HCl.
- While you were titrating, some NaOH dripped out onto the table instead of the flask.
- What is meant by “priming” the glassware? Why is this done?
- Why did you use burets instead of graduated cylinders to do this lab?
- Why did you not have to rinse out the flask in between trials?
- What is the pH of 0.100 M HCl (used in the lab)? Show your work.
- What is the pH of the KOH used in the lab?Show your work.
- Milk of magnesia (Mg(OH)2) is approximately 0.25M. How much 0.11M stomach acid (HCl) will be neutralized by 12.2 mL of milk of magnesia? Show all work.
- What volume of 9.2 M NaOH can be neutralized by 39 mL of 12.1 M HCl? Show all work.
- Complete the statement: Doing a titration without an indicator is like…. (I’m not looking for a right answer, just one that makes sense. Extra credit may be rewarded for creativity and sensibility.)