All chemists must know how to prepare solutions of varying concentrations because many reactions in quantitative analysis take place in an aqueous medium. In this experiment, solutions were made using 2 methods.
The first method used was dissolution. A specific mass of solid was dissolved in a specific volume of distilled water.
The second method was dilution. A specific volume of a certain concentration was added to distilled water until the desired concentration was obtained.
Once the standard solutions have been prepared, they were standardized. Standardization involves preparing a solution using the primary standard and titrating it with the titrant to the endpoint to be able to calculate the exact concentration of the primary standard.
PROCEDURE OF THE EXPERIMENT
There are four parts to this experiment, two of which involve preparation with the remaining to involving standardization.
The first part was the preparation of 250ml of 1.0 M sodium hydroxide (NaOH) solution from a solid. Using a top-loading balance, ____ g of NaOH was transferred to a clean Erlenmeyer flask. Distilled water was then added until the solid was completely dissolved. The solution was transferred to a 250ml volumetric flask and mixed. Once mixed, the solution was bulked to the mark and mixed again. Finally, the solution was transferred to a clean plastic bottle and labeled.
The second part was the preparation of ml 100ml of 3.0 M hydrochloric acid (HCl) solution from a more concentrated (12.1 M) HCl solution. Using a pipette, ____ ml of concentrated HCl was transferred to a clean Erlenmeyer flask that already contained around 25ml distilled water. Distilled water was then added until the solution in the Erlenmeyer flask was around 90ml. The solution was transferred to a 250ml volumetric flask and mixed. Once mixed, the solution was bulked to the mark and mixed again. Finally, the solution was transferred to a clean plastic bottle and labeled.
The third part of the experiment was the standardization of 3.0 M HCl. Three clean Erlenmeyer flasks each had 0.1g of the primary standard Na2CO3 placed in them, along with approximately 50ml distilled water. The Erlenmeyer flasks were swirled to dissolve the solids. 2 drops of phenolphthalein indicator was added. Each solution was then titrated to the endpoint, using the previously prepared 3.0 M HCl solution as a titrant.
The fourth part of the experiment was the standardization of 1.0 M NaOH. Three clean Erlenmeyer flasks each had 0.1g of the primary standard KHP placed in them, along with approximately 50ml distilled water. The Erlenmeyer flasks were swirled to dissolve the solids. 2 drops of phenolphthalein indicator was added. Each solution was then titrated to the endpoint, using the previously prepared 1.0 M NaOH solution as a titrant.
Table 1. Reported Value for Titration of NaOH Solution
TrialWeight of Primary StandardNet Volume of NaOH
10.10009 g0.75 ml
20.0998 g1.30 ml
30.0995 g0.75 ml
Table 2. Reported Value for Titration of HCl Solution
TrialWeight of Primary StandardNet Volume of NaOH
11.048 g48.2 ml
20.0996 g53.69 ml
30.0994 g58.89 ml
PRINCIPLES OF THE EXPERIMENT
A specific amount of substance A will react with specific substance B in a predictable manner. If the amount of substance A in a solution is unknown, then adding solution B until all of A has reacted will tell you how much of A there initially was. This is the basic principle of titration. In titration, solution A (the analyte) is unknown. An indicator is added to it, which will participate in the titration reaction and undergo a visual change near the equivalence point. Solution B (the titrant) is then added in small increments until the end point is reached. Because the concentration of solution B is known, and the amount of substance B that will react with substance A is also known, the amount of substance A in solution A can now be calculated. In standardization, the inverse is applied. The amount of substance A in solution A is known. It is the concentration of substance B in solution B that is unknown.
Table 3. Calculated Values of Molarity of Solutions
TrialM NaOH% DeviationM HCl% Deviation
10.66 M34.46 %0.0408 M98.64 %
20.38 M62.41 %0.0348 M98.84 %
30.64 M35.04 %0.0317 M98.94 %
Mean0.56 M43.97 %0.0358 M98.81 %
SIGNIFICANCE OF THE RESULTS
The high percentage of percent deviation of all results from the theoretical value indicates that there is a systemic error.
Standardization using titration is an effective tool for testing if the concentration of a solution in practice matches the theoretical
ANSWERS TO QUESTIONS
1.It was possible to use phenolphthalein as an indicator for both solutions because Phenolphthalein changes from colorless to pink when there are more OH- ions than H+ ions (and vice versa). 2.Based on this experiment, the accuracy of solid reagents is better than the accuracy of liquid reagents. 3.There are many possible sources of error in this experiment, but the large discrepancy between the theoretical values and the results of the experiment suggest either a major human error, or a systemic error, possibly involving the weighing apparatus of the primary standards.
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