1 1. Abstract Acetic acid, CH 3 COOH is an organic compound that is in form of colorless solution and classified as a weak acid. Acetic acid is the main component ofvinegar apart from water. In this experiment, the molarity of a solution and the percent by mass of acetic acid in vinegar is determined by using titration with a standardized sodium hydroxide, NaOH solution. The experiment is divided into two parts which are standardizing the NaOH solution is the first part and the second part is proceeded with the determinin g the molarity of a solution and the percent by mass of acetic acid in vinegar. In standardizing the NaOH solution, 250 mL of distilled water is used to dilute approximately 6 g of NaOH solid in order to prepare 0.6 M NaOH solution. This NaOH solution is then titrated with potassium hydrogen phthalate, KHC 8 H 4 O 4 or KHP solution which has been prepared by diluting 1.5 g of KHP granules in 30 mL of distilled water. The experiment is then preceded to the second part of the experiment which standardized NaOH solution is titrated the with 10 mL vinegar that has been diluted with 100 mL of distilled water. Both titration for part 1 and 2 are repeated thrice to get more accurate results. Based on results, it can be conclude that the greater the mass of solute in the acid solution, the more concentrated the solution becomes thus, the higher the molarity and more volume of NaOH needed to neutralize the acid. The experiment is completed and successfully conducted. 2. Introduction Concentration of solution is the amount of solute in a given amount of solvent. A concentrated solution contains a relatively large quantity of solute in a given amount of solvent. Dilute solutions contains a relatively little solute in a given amount of solvent. There are two specific terms to express concentration, namely molarity and percent by mass: Molarity is the number of moles of solute per litre of solution. Molarity (M) = moles of solute Litre of solution (Equation 2–1)Percent by mass is the mass in grams of solute per 100 grams of solution Percent solute = grams of soluteX 100% Grams of solution (Equation 2–1)Vinegar is a dilute solution of acetic acid. The molecular formula for acetic acid is CH 3 COOH. Both molarity and percent by mass of acetic acid in the vinegar solution can be determined by performing a titration. A titration is a process in which small increments of a
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86725525 Determination of the Concentration of Acetic Acid in Vinegar
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8/20/2019 86725525 Determination of the Concentration of Acetic Acid in Vinegar
The sudden change in the solution pH shows that the titration has reached the equivalence
point. pH in an aqueous solution is related to its hydrogen in concentration. Symbolically, the
hydrogen ion concentration is written as [H3O+]. pH is defined as the negative of the
logarithm of the hydrogen ion concentration.
pH = – log10 [H3O+] (Equation 4 – 2)
pH scale is a method of expressing the acidity or basicity of a solution. Solutions with pH < 7
are acidic, pH = 7 are neutral, and pH > 7 are basic as shown in Figure 4.2. For example,, a
solution having an H3O+ concentration of 2.35 x 10 –2 M would have a pH of 1.629 and is
acidic. pH electrodes will be used in this experiment.
Figure 4.2: pH scale
The titration is initiated by inserting a pH electrode into a beaker containing the acid solution(pH 3 – 5). As sodium hydroxide, NaOH, is incrementally added to the acid solutions, some
of the hydrogen ions will be neutralized. As the hydrogen ion concentrated decreases, the
pH of the solution will gradually increase. When sufficient NaOH is added to completely
neutralize the acid (most of H3O+ ions are removed from the solution), the next drop of
NaOH added will cause a sudden sharp increase in pH as shown in Figure 4.3. The volume
of based required to completely neutralized the acid is determined at the equivalent point of
titration.
8/20/2019 86725525 Determination of the Concentration of Acetic Acid in Vinegar
Figure 4.3: Acid – base titration curve of weak acid titrated with NaOH
In this experiment, titration of a vinegar sample with standardized sodium hydroxide solution
will be performed. To standardize the sodium hydroxide solution, a primary standard acid
solution is initially prepared. In general, primary standard solutions are produce by dissolving
a weighed quantity of pure acid or base in a known volume of solution. Primary standard
acid or bases have several common characteristics:
They must be available in at least 99.9 purity
They must have a high molar mass to minimize error in weighing
They must be stable upon heating
They must be soluble in the solvent of interest
Potassium hydrogen phthalate, KHC8H4O4, and oxalic acid, (COOH)2, are common primary
standard acids. Sodium carbonate, Na2CO3, is the most commonly used base. Most acids
and bases (e.g. HCL, CH3COOH, NaOH, and KOH) are mostly available in primary standard
form. To standardize one of these acidic or basic solutions, titration of the solution with a
primary standard should be performed. In this experiment, NaOH solution will be titrated withpotassium hydrogen phathalate (KHP). The reaction equation for this is:
6.1 Standardization of sodium hydroxide, NaOH solution
1. Weight of NaOH solid needed to prepare the 0.6 M NaOH solution is calculated and
confirmed by the lecturer.
2. 250 mL of approximately 0.6 M NaOH solution is prepared from the NaOH solid.
3. A beaker is placed on the weighing balance and the beaker’s weight is tarred and
recorded.
4. Then, 1.5 g of KHP granules is added into the beaker and its weight is recorded to
the nearest 0.001 g.
5. Next, 30 mL of distilled water is added into the beaker and the solution is stirred until
the KHP granules completely dissolved in the solution.
6. The pH value of the KHP solution without the addition of NaOH solution is recorded
until pH meter reading stabilized.
7. This solution is then titrated with 2 mL of NaOH solution.
8. Then, the pH value of the solution is recorded after the reading on pH meter is
stabilized.
9. Step 7 and 8 are repeated until the pH values of the solution achieve its stable state.
10. After that, the titration process is repeated twice for NaOH standardization.11. The graph of pH versus NaOH for all titration processes are plotted and the volume
of NaOH needed to neutralize the KHP solution is determined from all three graphs.
12. Next, the molarity of NaOH solution for titration 1, 2, and 3 are calculated.
13. Lastly, the average molarity of the NaOH solution is calculated.
6.2 Molarity of acetic acid and mass percent in vinegar
1. 10 mL of vinegar is transferred into a clean and dry beaker.2. 100 mL of distilled water is then added into the beaker.
3. Next, pH value of the vinegar solution is recorded after the pH meter reading
stabilized.
4. Then, 2 mL of NaOH solution is added into the vinegar solution and its pH value is
recorded after the pH meter reading stabilized.
5. Step 4 is repeated until the pH values of the solution achieve its stable state.
6. After that, step 1 until 5 are repeated twice more for standardization.
8/20/2019 86725525 Determination of the Concentration of Acetic Acid in Vinegar
The purpose of this experiment is achieved as the molarity of the solution and the percent by
mass of acetic acid in vinegar have been determined by using titration with a standardized
sodium hydroxide solution. In the first part of this experiment in which to standardize the
sodium hydroxide solution, a primary standard acid solution is initially prepared. Sodium
hydroxide, NaOH is used as the base meanwhile potassium hydrogen phthalate, KHC 8H4O4
or jotted as KHP is used as primary standard acid. In addition, pH meter is used to measure
the pH of the solution. It is necessary to constantly measure and record the pH of the
solution by keep on immersing the pH meter in the solution to ensure more accurate results.
Based on graph and calculated result, it is showed that the average volume of NaOH
needed to neutralize the primary standard acid is 13.07 mL at average pH of 9.4. As mention
in the theory, solutions with pH less than 7 are acidic, pH equal to 7 are neutral, and pH
greater than 7 are basic. However, the result in this experiment does not exactly parallel with
the theory as the solution gains its equivalence point at average pH of 9.4. In other words,
the solution started to neutralize from pH of 6.208 up to 12.181. This is because of some of
the hydrogen ions are gradually neutralized with the increment volume of NaOH. Thus, a
sudden sharp increase in pH occurred as sufficient volume of NaOH is added into the acid
solution. Furthermore, the pH at the endpoint of a weak acid –strong base titration is alwaysgreater than 7 because strong base allows hydrogen ions in weak acid to neutralize more
easily.
The sudden fluctuation in the pH also occurred at the second part of this experiment. In the
second part of the experiment, the molarity of acetic acid and the mass percent in vinegar
are determined. The molarity of acetic acid and the mass percent in vinegar are calculated
by using the average volume of NaOH resulted from the first part of the experiment and with
the help of graph plotted based on results from the second part of the experiment. Theaverage molarity of acetic acid in vinegar is 0.9486 and its average percent by mass is
5.7873 %. In addition, it is important to dilute the vinegar in order to avoid a very small titre,
which would reduce the accuracy of the experiment.
The significance of percent by mass and molarity of solution in this experiment is that it tells
whether the solution is either diluted or concentrated solution. Hence, the acetic acid in the
vinegar is a dilute solution as its percent by mass and molarity are relatively small.
8/20/2019 86725525 Determination of the Concentration of Acetic Acid in Vinegar