How Pale Can You Go: Neutralization of HCl with NaOH
Last edited by Koni Stone on September 10, 2004

We will react hydrochloric acid with sodium hydroxide.  According to equation 1, one mole of HCl reacts with one mole of NaOH.  When enough NaOH has been added to react with all of the HCl the indicator, phenolphthalein will turn pink.  The paler the color of pink, the better.  Excess pink color means you added too much base and you should start over.  It usually takes several trials to get good results.  

HCl  + NaOH   ------->      NaCl + H₂O    (1)

If you know the number of moles of acid in the flask, and the volume of base added to reach the equivalence point, you can calculate the molarity of the sodium hydroxide solution. 

Moles of HCl = Molarity x Volume

Moles of HCl = Moles of NaOH  (How do we know this?, see equation 1)

Molarity of NaOH = Moles of NaOH/Volume

Note: Students work independently! This lab is graded on accuracy and precision, do everything very carefully.  Please show your instructor your pale pink colors.

Stockroom: Things to borrow and return on the same day.

	250 ml vol flask (~$20)
		25 ml pipet (~$15)
		Pipet pump
		Buret and brush (~$75)

Methods

1. You will want to carefully read the tips and hints for cleaning glassware that are provided by John Burt.  Clean glassware is essential for success in this laboratory exercise.

2. Titration of NaOH Solution.

From the 6 M NaOH solution available in the lab and distilled water, prepare 1 liter of 0.1 M NaOH solution. Store this in a polyethylene bottle or in a stoppered flask; the plastic container is preferred since basic solutions slowly react with glass. Under no circumstances should a volumetric flask be used. Be sure the solution is well mixed before you use it. At this point you only know that the concentration is approximately 0.1 M. Determine the exact concentration of the solution as described below.

Rinse a buret twice with approximately 10 ml of the 0.1 M NaOH solution and drain the solution through the buret tip. Then fill the buret with the 0.1 M NaOH solution; make sure there are no air bubbles in the tip of the buret or just above the stopcock. Run base out of the buret until the level is at 0.00 or below. Record the level of the base, estimating the reading to the nearest 0.01 ml.

Pipet a 25.00 ml aliquot of the HCl solution into a 200 or 250 ml Erlenmeyer flask. Dilute with about 25 ml of water (use your wash bottle), washing down the sides of the flask in the process. Add 2 drops of phenolphthalein indicator solution.  Write down the molarity of the HCl solution in your notebook.

Slowly run base out of the buret into the HCl solution, swirling the flask and contents. As you approach the end point, the area in the HCl solution where the drop of NaOH falls will turn pink; then the pink color will disappear as the solution becomes mixed. From this point on, add the NaOH drop wise with constant swirling. Occasionally wash down the sides of the flask with water from your wash bottle. The end point is where 1 drop (or less) of NaOH solution causes the solution to become permanently pink throughout. Essential ingredients for a successful titration in include care and patience, so don't try to hurry. Record the final buret reading, estimating it to the nearest 0.01 ml.

Sample Buret Reading

Repeat the titration two or three more times using a clean flask each time. After the first titration, the others should go more quickly since you now have some idea of how much base is required per aliquot of HCl solution. The base may be added quickly until you are within 2 or 3 ml of the end point; then change to drop wise addition. Record your data in your notebook. A proposed format is attached for you to consider in recording your data.

Wastes and Spills: All the chemicals in this experiment can be put in the trash or down the drain. Be sure the water is running and be careful with the 6 M NaOH. Use wet paper towels to wipe up small spills. Rinse the pipet and buret and other glassware with water before returning them to the Stockroom.

Data analysis

Organize your data in tables using  and be sure to include a copy of the table showing the formulas that you used. You must show a sample of every calculation.

Calculate the molarity of your NaOH solution for each titration, and report an average molarity.

For every average that you report, calculate and report an average deviation.

Conclusion

Restate all of your results. Be sure to report average values ± average deviations.

Answer the following questions in well organized paragraph form:

List the possible sources of error, and evaluate your performance with regard to each of these sources. Describe exactly how these errors would affect your calculated values for the molarity of the NaOH solution. (Would the determined values be higher or lower than the actual values?)

Define accuracy and precision. Discuss your results in terms of accuracy and precision.