Purpose: To determine the concentration of a solution of hydrochloric acid using a primary standard (solid Na₂CO₃) and a secondary standard (aqueous, standardized NaOH).

Procedure:

Preparation of a solution of hydrochloric acid

1. Weigh out 0.25 ± .05 g of dry Na₂CO₃. Record actual mass used to nearest .001 g. Transfer the solid to a 250 mL Erlenmeyer flask, rinse the weigh paper into the flask with a small amount of de-ionized water and dissolve the solid with 25 mL of de-ionized water. Add 3 drops of bromcresol green once the solid is dissolved.
2. Clean and check the flow rate of a buret (as described in the handbook).
3. Rinse the buret 3 times with 5 mL of the 0.1M HCl solution to be titrated. Discard the rinses. With the stopcock open and over a beaker, begin filling the buret with 0.1M HCl solution. Have your lab partner close the stopcock once the tip is draining and the volume is above the stopcock. This will prevent bubbles from forming in the stopcock during the filling process. Finish filling the buret to within a few mL of the zero mark. Record this volume as the initial volume in the titration. (Note that this is NOT the volume of liquid in the buret.)
4. Begin titrating the sodium carbonate solution with the hydrochloric acid solution by adding small increments of the titrant from the buret. Swirl the reaction mixture between additions. Use smaller increments of HCl as you get closer to the expected endpoint (Pre-Lab question 2). The acid-base indicator, bromcresol green, will eventually turn from blue, which indicates an excess of Na₂CO₃, to yellow, which indicates an excess of HCl. The endpoint will be when the solution remains green, which indicates that the stoichiometric quantity of HCl has been added.
5. Once the solution is green, bring it to a gentle boil on a hot plate to expel all dissolved carbon dioxide. Remove it from the hot plate to a wire gauze, and rinse the inside surfaces with a small amount of de-ionized water. If the solution stays green, the titration is complete. Record this volume as the final volume of titrant. If it turns blue, allow it to cool, and complete the titration by adding a few drops more of titrant. If the solution is yellow, you have overshot the endpoint and must repeat the titration.

1. Pipet 25 mL of the HCl solution into a 250-mL Erlenmeyer flask. Add 50 mL of de-ionized water and 3 drops of phenolphthalein.
2. Rinse the buret 3 times with 10 mL of the standard NaOH solution. Discard the rinses. With the stopcock open and over a beaker, begin filling the buret with the NaOH solution. Have your lab partner close the stopcock once the tip is draining and the volume is above the stopcock. This will prevent bubbles from forming in the stopcock during the filling process. Finish filling the buret to near the zero mark. Record the precise concentration of the standard NaOH solution and the V_initial in your notebook.

Compare the molarity determined using a primary standard and the molarity determined using a secondary standard. Which molarity do you think is the most reliable and why? Use this molarity as your accepted value to calculate the %error.

Pre Laboratory Assignment

• Read section VIII of your Laboratory Handbook for information on the use of volumetric glassware, making solutions and pipetting.
• Prepare a table in your laboratory notebook to put your data in. Make sure that you have thought about all of the data you will need and leave a place for your observations. Show it to your instructor as you walk in the door. Make sure that he/she signs it. (What kind of data do you need? Initial and final buret readings. Mass of sodium carbonate and lots more!
• Read Chapter 5, Section 6 (Section 9 in 4th Ed.) of the textbook to learn more about titrations.

1. Calculate the approximate concentration of a hydrochloric acid solution prepared by diluting 100 mL of 12.3M HCl to 2.00 liter.
1. Calculate the approximate volume of 0.095 M HCl needed to neutralize 0.25g of sodium carbonate. Hint: Write a balanced equation first.
1. Calculate the approximate volume of 0.0879 M NaOH needed to neutralize 25.0 mL of 0.095 M HCl. Hint: Write a balanced equation first.
1. Write an equation (including states) for the reaction between carbon dioxide gas and water. Why is this reaction significant in this experiment?
2. A student failed to rinse a damp buret with a small amount of the acid before filling it with acid. (p 23 lab handbook). Will more or less titrant (acid) be required to neutralize the sodium carbonate?

Purpose: What you are doing in the lab. The question you are answering.

Procedure & apparatus: A description of the procedure. This can be simply the statement: "As given in the lab handout." It should include any modifications to the experiment.

Data: This section contains all observations, including tables with measurements, etc.

Calculations: This section must show samples of all of the different types of calculations. Include any graphs in this section. Show at least one sample calculation for the Na₂CO₃ titration and one calculation for the NaOH calculation. You may feel free to show the entire square method.

Discussion and errors. A section that describes your results and how good you think your results are. (For example: Although the density of my metal is close to the literature value for gold my results varied widely and the sample did not look like gold.) Although you may have more, you must describe at least 4 sources of error.

The first line in this section could be "My data is good (or bad) because ……)

Another paragraph should be "My errors include..

1.

2.
 
 
 
 

Modifications: Describe at least one modification you would make to improve the accuracy of the procedure. (Presumably it fixes one of the errors you described.)

Conclusion: It is the answer to the problem posed in the purpose In this case, " The concentration of the HCl was _____ from a ....

Answers to the post laboratory questions: Answers to the post laboratory questions.