Lab Practical: Titrating an Acid or a Base Name:______
PROBLEM: How do you measure the concentration of an acid or base?
During a neutralization reaction, an acid and base react to produce a salt and water.
Acid + Base à Salt + Water
It is easy to tell when an acid completely neutralizes a base by using an indicator that changes color at, or close to, the pH at the equivalence point for the titration. The equivalence point is the point at which the moles of hydronium ions from the acid just equal the moles of hydroxide ions from the base. For strong acid –strong base titrations, the pH at the equivalence point is 7.
The indicator phenolphthalein has a characteristic color in solutions with a pH less than 8 (what color? Table M). If enough base is added to the acid, the phenolphthalein changes color (To what color?).
If the volume of a base (of known concentration), needed to neutralize a known volume of an acid (of unknown concentration), is measured, then the concentration of the acid can be calculated by the following relationship:
Ma × Va = Mb × Vb
A neutralization reaction performed in this manner to determine the concentration of an acid or base is called a titration. In this laboratory investigation, you will determine the concentration of an acid by titrating it with a base of known concentration.
MATERIALS
Beaker[250 mL], burette clamp, burette, 50 mL conical (Erlenmeyer) flask, graduated cylinder, hydrochloric acid [unknown concentration], pipette, phenolphthalein, ring stand, sodium hydroxide solution [approximately 0.1 mol/L; The exact concentration of NaOH will be given to you by your teacher – Be sure to record this concentration ]
PROCEDURE
1. With a graduated cylinder, add 10.0 mL of hydrochloric acid to a clean 50 mL conical flask. Add two drops of phenolphthalein indicator. Note the color. Record the volume of the acid in the data table on the next page. Label this flask and move it off to the side of your work space until you have completed steps 2-5.
2. Set up the stand with a burette clamp and burette as shown in the diagram to the right. Use a 250 mL beaker for the cleaning process (step 3).
3. Place the 250 mL beaker under the burette, close the stopcock and run some de-ionized water into the top of the burette. Let the water clean the inside of the burette. Open the stopcock, and allow the water to drain out. Repeat.
4. Close the stopcock, and, using a funnel, pour some of the sodium hydroxide solution of known concentration into the burette (about 20 mL). Open the stopcock, and allow the solution to drain out into your waste beaker.
5. Make sure the stopcock on the burette is closed. Using the sodium hydroxide from the bottle provided, fill the burette about 1cm past the zero mark (the top mark) with base. Put the 250 mL beaker under the burette. Open the tap slowly and run enough NaOH (aq) from the burette to remove air from the tip of the burette and to bring the base level in the burette down to the graduated portion of the burette.
6. Examine the fluid level in the burette. Locate the meniscus. Record the initial volume of the NaOH(aq) in the data table on the next page. Record the exact concentration of the sodium hydroxide in your table.
7. Remove the beaker and position the conical flask with the acid and indicator under the tip of the burette. Hold the flask containing the acid under the burette. Run base slowly into the acid, mixing by swirling the flask.
8. When the color of the acid begins to change, add the base one drop at a time until one final drop causes a complete and permanent color change. Note the color.
9. Record the final volume of the base in the burette in the data table on the next page.
10. Calculate the amount of base used by subtracting the initial volume from the final volume. Record the result.
Determine the concentration of the acid using the relationship Ma × Va = Mb × Vb.
Include sample calculation below. (10 pts)
Repeat step 1 and steps 6 through step 10 two more times for a total of three trials. If necessary, add more base to the burette and record a new initial volume.
Find the average concentration of the base by adding the results of the three trials and dividing by 3. Include calculation below. (10 pts)
Grade for evaluation of safety procedures , lab techniques and clean up. /25 pts
Data Table (30 pts total)
Volume (mL) / First Trial / Second Trial / Third TrialVolume of acid (mL) /
Volume of base (mL) / Initial volume /
Final volume /
Amount used /
Known concentration of base(mol/L) /
Calculated concentration of acid (mol/L) /
Average concentration of the acid
(mol/L)
Questions (25 pts total)
1. What color is phenolphthalein in acid? in base?
______(2)
2. Write a chemical equation showing the neutralization of sodium hydroxide with hydrochloric acid.
______(2)
3. What is the purpose of doing three trials?
______(2)
______
4. If 20 mL of sodium hydroxide are needed to neutralize 40 mL of 3 M hydrochloric acid, what is the concentration of the sodium hydroxide? Show work. (3)
5. How do you measure the concentration of an acid or base?
______(2)
______
Base your answers to questions 6 through 8 on the information below.
In a titration, a few drops of an indicator are added to a flask containing 35.0 milliliters of HNO3(aq) of unknown concentration. After 30.0 milliliters of 0.15 M NaOH(aq) solution is slowly added to the flask, the indicator changes color, showing the acid is neutralized.
6. The volume of the NaOH(aq) solution is expressed to what number of significant figures?
______(2)
7. Write a fully balanced chemical equation for this neutralization reaction
(4)
9. Calculate the molarity of the HCl(aq). Your response must include both a numerical setup and the calculated result.
(7)
10. According to the data, to what number of significant figures should the calculated molarity of the HCl(aq) be expressed?
(1)