Lab 12: Acid/Base Neutralization and Titration
Background:
The neutralization reaction and the presence of an acid/base indicator can be used to experimentally
determine the concentration of either a base or an acid. The solution of known concentration, called the standard
solution, is used to neutralize a precisely measured volume of solution of unknown concentration to which one or
two drops of an appropriate acid-base indicator has been added. A sudden change in color of the indicator
signals when the solution has been neutralized - the endpoint. At the endpoint, the number of hydrogen ions from
the acid is equal to number of hydroxide ions from the base. When the endpoint of the titration has been reached,
the volume of standard solution and the solution of unknown concentration is carefully determined. The volume
of both solutions and the concentration of the standard solution are used to calculate the concentration of the
other solution.
Purpose:
To practice titration and stoichiometry
Procedure:
A. Wash and rinse a buret.
B. Rinse the inside surface of one buret with 10 mL of the standard solution.
Allow the solution to flow out the tip.
Fill the buret to above 0.0 mL mark.
Release enough solution to remove air bubbles and lower the volume to 0.0 mL.
Record this volume as the initial volume of standard solution.
C. Add exactly 10.0 mL of solution to be titrated to a 125 mL Erlenmeyer flask.
Record this volume as the volume of the solution with unknown concentration.
Place the 125 mL Erlenmeyer flask beneath the base buret.
Add 3 drops of indicator to the solution.
D. Swirling the flask gently, begin the titration by slowly adding standard solution.
Continue until the neutral end-point has been reached.
If over-titration occurs, add more of the unknown drop by drop.
E. Record the final volume of both the acid and base added to reach neutrality.
F. Repeat the titration.
Analysis of the Reaction:
1. Write the balanced equation for the neutralization of the acid and base.
2. Use a collision model and bonding to explain the reaction between the acid and base.
Compare the attractions in the solutions and the new bonds that are formed.
Explain the formation of water in the neutralization reaction and changes in pH.
3. Explain the reversible ionization of water that occurs in the neutral solution,
as well as the basic and acidic solutions.
a. Write the equation for the ionization of water.
b. Write the equation used to calculate the value of Keq or Kw for this reaction.
c. Write an expression to show the concentrations of the hydrogen and hydroxide ions in a neutral
solution. How do the concentrations compare?
d. Compare the acid and base solution- what is the Kw for both? Compare the concentrations of
hydrogen and hydroxide ions in both.
e. What happens to value of Kw when the acid is neutralized by the base?
What happens to the concentrations of hydrogen and hydroxide ions in the acid?
Explain these changes in terms of equilibrium and the value of Kw.
4. If the concentration of the base (NaOH) is 0.010 M.
a. What is the concentration of the OH- ion?
b. What is the concentration of H+ ion?
c. What is the pH?
d. When you add acid to this base, what happens to the pH and ion concentrations?
Calculations:
5. Moles of hydrogen and hydroxide ions at neutrality
a. The standard solution is the solution of known concentration. Which solution was the standard solution
in your titration?
b. Since the solutions are mixed until neutral, what must be true about the number of hydrogen ions from
the acid compared to the number hydroxide ions from the base?
c. So, if your standard solution is an acid, once you calculate how many ions of hydrogen you added, you
know the same number of hydroxide ions must be in the base. Make a similar statement that must be
true if your standard solution were a base.
d. Calculate the number of moles of hydroxide ions or number of moles of hydrogen ions (depending upon
whether your standard was acid or base) added from the standard solution to the mix of the acid and
base when it’s neutral. If the standard solution is an acid- calculate the moles of hydrogen ions in the
standard solution. If the standard solution is a base, calculate the moles of hydroxide ion in the
standard solution.
e. State the relationship between number of hydrogen and hydroxide ions in the neutral solution.
f. State the number of hydrogen or hydroxide ions that you calculated must have been added from the
standard solution. Therefore, state the number of hydroxide or hydrogen ions that must have been
added from the other solution. Use your understanding of neutrality to justify your response.
6. Calculate the molarity concentration of the solution of unknown concentration. Show the complete
calculation using stoichiometry. Think: What are the units of molarity?
7. How reproducible (precision) were your results?
Compare your calculations of the concentrations based on two trials.
Compare your two trials with two trials from another lab team.
8. How accurate are your results? Calculate the percent error based upon the accepted concentration of the
unknown solution?
Other Questions:
9. If 30.0 mL of 0.500 M KOH are need to neutralize 10.0 mL of HCl of unknown concentration, what is the
molarity of the HCl? Write balanced equation and show complete calculation.
10. If 39.5 ml of 2.0 M HC2H3O2 are added to 15.0 mL of Mg(OH)2, what is the concentration of the
magnesium hydroxide solution? Write balanced equation and show complete calculation.
11. Name the acid- and write the ionization reaction (as either reversible or to completion)
and Ka expression and its value where appropriate (see page 557 Table 15-4)
a. with flourine- weak acid Ka = 7.1 x 10-4
b. with nitrate- strong acid
c. with chlorine- strong acid
d. with sulfite- weak acid
e. with sulfate- strong acid
f. with nitrite- weak acid
g. with acetate- weak acid
h. with phosphate- weak acid
12. pH and concentrationa. A solution has a pH of 5. Is it an acid or base? What is the concentration of hydrogen and hydroxide
ions?
b. A solution has a pH of 10. Is it an acid or base? What is the concentration of hydrogen and hydroxide
ions?
c. If the pH increases from 6 to 8, what happens to the hydrogen ion concentration? (increase or
decrease by what factor) What happens to OH- ion concentration?
13. A vinegar sample is found to be 0.837 M acetic acid. Its hydronium ion concentration is measured as
8.86 x 10 -3 M. Calculate the Ka of acetic acid.