EXPERIMENT 9

[vinegar.lab]

OBJECTIVES:

1. To learn to use a PIPET.

2. To be able to read the volume of liquid in a BURET.

3. To be able to TITRATE an acid with a base until the two are equivalent (ie: neutral).

4. To be able to calculate the percent of acid in a sample of an unknown acid ("vinegar").

DISCUSSION:

The concentration of solutions is commonly expressed as MOLARITY. A 1.0 M solution contains 1 mole of a substance in 1 liter. A 0.001 M solution contains 0.001 mole per liter. To find molarity, we must know the molarity of another solution, called a STANDARD SOLUTION, or use a solid PRIMARY STANDARD of known purity. In this experiment, you will be given a standard solution and then use this to find the concentration of other solutions.

You will prepare a dilute solution of NaOH and then find its molarity by TITRATING it with the standard solution. You will then use the NaOH to find the molarity and percentage of a "vinegar" solution. TITRATION means to add a base to an acid until the two have equal numbers of molecules and the solution is "neutral".

To help us decide when the solution is neutral, we will use an indicator. Phenolphthalein (PP) is colorless in acids and red in bases. You will measure an exact amount of acid (using a pipet), add the indicator and then add the base from a buret until the indicator becomes, and remains, a light pink. From the volumes of acid and base used, you will then calculate the molarities, and finally the percentage of vinegar.

EXPERIMENTAL PROCEDURE: Work as individuals.

Part A: DETERMINING THE MOLARITY OF THE BASE

1. Record the molarity of the Standard Acid (HCl) as M_a.

2. Pipet exactly 25.00 mL of Standard Acid solution into a clean 125 mL erlenmeyer flask. Record 25.00 mL as V_a. Add 2-3 drops of PP.

3. a. Add about 480 mL water to a clean 600 mL beaker. Add about 17 mL of 3 M NaOH. Stir the solution well. If any NaOH is spilled, clean it up immediately with water.

b. Fill the buret with this NaOH solution. Let it run out until it is between 0.00 and 1.00 mL. Record this initial volume as V_A1.

4. Add the NaOH in the buret to the Acid until a PINK color appears and remains. (We want a pale PINK, not a RED color). This is called TITRATING the acid with the base. Read the volume of NaOH left and record as V_A2 (final volume). Subtract V_A1 from V_A2 to get the difference V_A12.

5. Rinse out the flask. It should not be dried. Repeat the titration once more. If the second volume is within 0.10 mL, you may stop. If the volumes are different, repeat again. Average the "good" volumes by adding the B values and dividing by the number of values. Label this average V_A12. (If you have more than two trials, the Instructor will help you decide which are the "good" volumes.)

6. From the molarity of the Acid and the volumes of the Acid and the NaOH, calculate the exact molarity of the NaOH solution, using the equations given below and on the data sheet.

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Part B: TITRATING THE VINEGAR

7. Get a sample of vinegar from the instructor. Fill your pipet half-full with this vinegar, shake and let it drain out. This will remove the old acid solution.

8. Pipet 25.00 mL of the vinegar into the erlenmeyer flask. Add 2-3 drops of PP.

9. Titrate the vinegar with the NaOH solution until you get a pink color. Repeat this until your results agree within 0.05 mL. Average the volumes of NaOH used and label this average V_B12.

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Part C: CALCULATING THE PERCENT OF ACETIC ACID IN VINEGAR

10. Vinegar is a mixture of water and acetic acid, which has the formula HC₂H₃O₂, or CH₃COOH. We will calculate the percentage (%) of acetic acid in the vinegar using the equation given below.

Using abbreviations for the variables, we see:

Rearranging this equation gives:

mg

11. Hand in the Lab Record. Rinse the pipet and buret and return them to the Instructor.

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KNOW the terms used in this experiment. These include:

titration buret primary standard

indicator pipet phenolphthalein

neutral molarity erlenmeyer flask

acid base standard solution

EXP 9 - LAB RECORD Name ____________________ Sec _____

ANALYSIS OF VINEGAR

Molarity of Acid is given as __________ M = M_a

Volume of Acid used is __________ mL = V_a

A. Determining the Molarity of the BASE (NaOH)

M_b = ( M_a * V_a ) / V_A12

M_b = ( __________ * __________ ) /__________

M_b = __________ M

B. Titration of VINEGAR

C. Calculating the percentage of Acetic Acid in the vinegar:

The acid in vinegar is acetic acid, CH₃COOH, so the molar mass (gfm) =

M_b is the molarity of the Base used, while V_B12 is the volume of the base used for the vinegar.

mg (milligrams) is the mass of the vinegar used. We used 25 mL. This is very close to 25 grams, or 25000 mg.

% acetic acid in vinegar = __________ %

Instructor's use:

Correct % = Error = Grade =