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Virtual Titration Lab


ACID - BASE TITRATION

PRE-LAB DISCUSSION:

In the chemistry laboratory, it is sometimes necessary to experimentally determine the concentration of an acid solution or a base solution. A procedure for making this kind of determination is called an ACID-BASE TITRATION. In this procedure, a solution of known concentration, called a STANDARD solution is used to neutralize a precisely measured volume of the solution of unknown concentration to which one or two drops of an indicator have been added. If the solution of unknown concentration is acidic, a standard base solution is added to the acid solution until it is neutralized. If the solution of unknown concentration is basic, a standard acid solution is added to the base solution until it is neutralized. In this lab, we will be finding the concentration of five unknown acid solutions. We will have a choice of any of three standard base solutions, 0.05 M NaOH, 0.10 M NaOH, and 0.40 M NaOH.

When carrying out an acid-base titration, you must be able to recognize when to stop adding the standard solution. That is, you must be able to recognize when neutralization has occurred. This is the purpose of the INDICATOR. A sudden color change due to the indicator signals that neutralization has occurred. At this point, the number of HYDROGEN IONS (H+) from the acid is equal to the number of HYDROXIDE IONS (OH-) from the base. The point at which this occurs is called the END-POINT of the titration. When the endpoint is reached, the volume of the standard solution is carefully determined. Then the measured volumes of the two solutions and the known concentration of the standard solution can be used to calculate the concentration of the other solution.

At the end-point of the experiment, the volume of the acid times the molarity of its hydrogen ions will equal the volume of the base times the molarity of its hydroxide ions. Calculations will be based on the following formula:

(volume of acid)(molarity of H+) = (volume of base)(molarity of OH-)
or, more simply…
VaMH+ = VbMOH-
Knowing three of these quantities we can calculate the fourth -- in this case the molarity of the acid's hydrogen ions. Since the acids and the base in this experiment have one hydrogen ion and one hydroxide ion, respectively, the molarity of hydrogen ion is the same as the molarity of the acid, and the molarity of hydroxide ion is the same as the molarity of the base.

PURPOSE:

To learn the experimental technique of titration and using this technique, to determine the molarity of five acid solutions of unknown concentration.

PROCEDURE:
  1. Follow the guidelines for lab write-ups. Your lab must be submitted handwritten or typed. It cannot be a printed version of this page. It will be graded according to the standards in the Lab Rubric.
  2. You are constrained to using 25.00 mL of each acid solution. Select "Acid A" from the pull-down menu and record "25.00 mL" for the volume of acid used in the table in Data and Observations.
  3. Select an indicator for the titration. Although indicators have different transition intervals (pH ranges over which they undergo color change), there will be no difference in your results in this lab. The only difference will be in the color change you observe at the point of neutralization.
  4. Select a concentration of NaOH to use as the Standard Solution, and record this concentration in the Data and Observations table. The more concentrated the base, the less you will have to add. However, increasing the concentration of the base also increases your chances of "over-titrating." Over-titration occurs when you add so much base that you go PAST the point of neutralization.
  5. Add base, initially using the 1.00 mL, and then moving to smaller quantities as you believe you are approaching neutralization. If you believe that you may have added too much base and over-titrated, then repeat the titration and begin using smaller amounts of base sooner. When you are confident that you know the EXACT amount of base needed to neutralize the acid, record the volume of NaOH used in your Data and Observations table.
  6. Repeat the procedure for the four remaining acid solutions: Acid B, Acid C, Acid D, and Acid E.

Flash simulation courtesy of UMass Chemland Chemistry.


Results

DATA AND OBSERVATIONS:


Acid A Titration Acid B Titration Acid C Titration Acid D Titration Acid E Titration
Volume of Acid (mL) mL mL mL mL mL
Molarity of Base (M) M M M M M
Volume of Base (mL) mL mL mL mL mL


CALCULATIONS

Showing ALL of your work, calculate the molarity of each of the acid solutions. Include both your work and results in a table organized like the one below.

Acid A Titration Acid B Titration Acid C Titration Acid D Titration Acid E Titration
Calculations
MH+ = (VbMOH-) ÷ Va		          
Molarity of Acid (M) M M M M M