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Standard Solution Essay

ABSTRACT

The purpose of this experiment is to prepare a standard solution of potassium hydrogen phthalate and use titration to perform an acid/base reaction between the potassium hydrogen phthalate and sodium hydroxide to standardize approximately 0.10 M sodium hydroxide solution. To prepare the Potassium Hydrogen phthalate, a 2.00 grams of KHP was measured to an accurate measurement of 1.980 grams. A total of 100 mL of water was mixed with the KHP solution in the volumetric flask to finally prepare an acidic KHP solution. The molar mass of KHP was calculated and came to be 208.252 grams per mole. To find the moles of KHP, the mass of KHP (1.980 g) was divided by the molar mass of KHP ( 208.252 g) and .00951 moles are in the 1.980 grams of KHP.

To find the molarity, the number of moles of KHP (.00951 moles) was divided by the volume of water in liters (.100 L) giving an answer of 0.0951 M. The second part of the experiment is the reaction of acid/base through titration. The acid, KHP, was placed in one burette while 100 mL of base, stock solution of NaOH, was placed in another burette. 25 mL of KHP was placed into the Erlenmeyer flask, which was also recorded as the volume of acid in the flask. A total of 19 mL of base, NaOH, was required for the endpoint, which was indicated by a very pale pink color. To calculate the molarity of NaOH, the following equation was used MNaOH x VNaOH = MKHP x VKHP therefore the molarity was .125 M.

INTRODUCTION

This lab experiment covers the preparation of standard solution and the acid/base titration. The first part of the lab is to prepare a standard solution of Potassium hydrogen per. A standard solution is a solution of known concentration, in which it is prepared using exacting techniques to make sure that the molarity is to the highest accuracy. The stock solution on the other hand, which is also a base in this experiment, is a large volume of a common reagent prepared to the 1 or 2 significant figures of accuracy. The stock solution in this experiment is 0.10 M Sodium Hydroxide. Standard solutions are only created when needed and is kept for a short time frame because the common reagents decay over time and collect additional water therefore changing the concentration as a function of time. Once a solution is standardized, this solution is used to quantitatively measure the number of chemicals in other solutions along with the chemicals used to react with the standard solution. The first method, which is used to prepare the potassium hydrogen phthalate, for preparing standard solution is to use solids that have a high stability in air because these can be weighed on a balance without losing or gaining mass.

An alternative method is preparing standard solutions to create a solution and than react this solution with a another standard solution. After the standard solution is prepared, part B is to create an acid/base reaction between potassium hydrogen phthalate and sodium hydroxide to standardize the approximately 0.10 M sodium hydroxide stock solution. Sodium Hydroxide cannot be used to prepare a standard solution through the first method because it is hygroscopic and readily absorbs water from air. Sodium hydroxide also reacts with carbon dioxide in air and can’t be stored for long periods of time therefore the stock solution of sodium hydroxide is standardized using the second standardization method. Acid/base reactions are neutralization reactions that create a salt and at times water.

The only method to accurately conduct acid/base neutralization reactions is titration. Titration is the precise measurement of volume of one reactant of known molarity required to completely react with another reactant of unknown molarity in a chemical reaction. When a reaction is done, the titration has reached its end point or also known as equivalence point therefore titrations are best used to figure out the accurate concentration of a reactant in a solution for various chemical reactions. In acid/base titration, the base of known molarity is added to an acid on unknown concentration or the acid of known molarity is added to the base of unknown concentration. For experiment, the acid of known molarity was added to the base of unknown concentration. The equivalence point indicates that a completely neutralization has taken place.

The neutralization indicates that the acid has reacted with the base to form a water and a salt. For KHP and NaOH, the neutralization reaction formed the aqueous sodium potassium phthalate and water. A way of determining that the equivalence point of acid/base titration is to use a weak acid phenolphthalein, which is colorless in acidic solutions but a pale pink color in neutral solutions and dark park for basic solutions. To determine the molarity of a solution from a acid/base titration, the following reaction is used MNaOH x VNaOH = MKHP x VKHP . Since the Molarity and Volume of KHP will be already determined and the volume will determined after the titration, the numbers are filled in and calculated to determine the MNaOH.

EXPERIMENTAL PROCEDURE

Part A

The first part of the experiment is the preparation of a standard potassium hydrogen phthalate (KHP) solution. A 100 mL volumetric flask was obtained form the stockroom and rinsed with 25 mL of distilled water. No droplets were formed on the inner surfaces of the flask. Two grams of the KHP solution were placed in a clean dry beaker, and weighed on the electronic balance. The exact weight of the KHP in grams was recorded. Next 20 mL of water was poured into the beaker, which will begin to dissolve the KHP. Afterwards the KHP was then carefully placed into the volumetric flask without spilling any of the solution. The beaker of ,which the KHP was formerly placed in, was washed with three aliquots of 10 mL of distilled water. These washings were then added to the volumetric flask, which ensured that any kHP stuck on the walls of the beaker is transferred to the volumetric flask. There was approximately 60 mL of distilled water present in the volumetric flask.

An indicator of this is the water level is 3/4 of the way up the bulb of the volumetric flask. The flask was closed with a lid specifically a parafilm and then was inverted several times to mix the contents.This process was continued until the KHP was completely dissolved. The parafilm was removed and rises with a small amount of distilled water to make sure that the water drops in the volumetric flask. Next distilled water was added to the flask enough to fill the flask to the etched line. The flask was covered with the lid and and inverted again several times to mix the contents. The flask cover was removed to make sure that all of the solution remained in the volumetric flask. The water was added with a clean medicine dropper to bring the bottom of the meniscus to the etched line. The flask was then shown and approved by the instructor and the volume was recorded onto the data table. Part B

The second part of the experiment is Titration of the acid and base. The acid is KHP and the base is NaOH (Sodium Hydroxide). First two burettes were obtained and setup on the lab bench by placing the stand on the lab bench and the burette clamps placed at a medium height on the stand. The two burettes were then held on right side of the clamp. Next a few mL of water was poured into the two burettes and the stopcock was tested by controlling the liquid to flow one drop at a time. Then a few mL of distilled water was poured into the burette and rolled around to wet the inside of the burette. It was then rinsed with tap water of three 10 mL portions allowing some of each potion to run through the tip and the same with 10 mL aliquots of distilled water. Once the burette was cleaned the burette was rinsed with thee 10 mL portions of the solution that the burette will hold. One burette contained 100 mL of 0.10 M sodium hydroxide solution and the other contained the standard kHP solution. In each case, some of the solution was allowed to run through the tip.

The rinsing were then removed in the appropriate waste container. The stopcock was checked to see if it was closed. The burette was held with its top below eye level and filled with the solution somewhere near the top mark. The filled burette was clamped in a vertical position and the initial volume of the liquid in each burette was recorded. before taking the volume reading, one must wait 15 seconds in order to make sure that the liquid has drained down the inner surface of the burette. 25 mL of KHP was poured from the burette containing KHP into a clean Erlenmeyer flask. The actual volume of the KHP solution added was recorded onto the data table. Next 25 mL of distilled water along with 2 drops of phenolphthalein was added to the solution in the flask. The Erlenmeyer flask was then placed on a sheet of white paper under the burette containing the sodium hydroxide solution. The sodium hydroxide was added to the KHP one drop at a time. After each drop, the flask was swirl gently to mix the acid and the base solutions thoroughly.

Then the sodium hydroxide solution was added at 1-2mL portions at a time. First a transient pink color will appear as the sodium hydroxide is added. As the titration continues, the pink color persists for longer intervals but disappears. When this happens, the sodium hydroxide was added in small portions. The longer the pink color persisted, the smaller the amount of sodium hydroxide was added. When the equivalence points got closer, the sodium hydroxide was added drop wise. The equivalence point is marked by the first permanent faint color in the KHP. Permanent is a color change that persists at least for 30 seconds. The pink color sometimes might fade away due to the absorption of CO2 from the air. The end point color was then showed to the professor. However the professor indicated that the equivalence point was overshot because the solution is a dark pink color. The KHP solution from the acid burette was added drop wise until the colored disappeared. The sodium hydroxide from the base hydroxide was added drop wise until the equivalence point is reached. The volume of KHP and the volume of the sodium hydroxide was recorded to the data table. After the titration was completed, the calculate value for molarity of the sodium hydroxide solution on the data table in the laboratory notebook. The burettes were then emptied and rinsed with three 10 mL portions of distilled water and returned to the stockroom. All other materials were also rinsed and put away.

RESULTS/DISCUSSION

Experiment A.

|Molar Mass of KHP |208.252 g/mol | |Mass of KHP |1.980 g | |Volume of Water |100 L | |Molarity of KHP |.0951 M |

For the first part of the experiment, the molar mass of KHP was calculated. The chemical composition of KHP is actually KHC8H8O4 therefore the molar mass was calculated. 39.10 g/moles (K) + 1.008 g/moles (H) + 96.08 g/moles (C8) + 8.064 g/moles (H8) +64 g/moles (O) = 208.252 grams. The 2 grams of KHP actually weighed 1.980 grams. Next 20 mL of water was added to the beaker to begin to dissolve the KHP, which was then transferred to the volumetric flask. The beaker was rinsed with the three aliquots of 10 mL of distilled water, which was added to the volumetric flask in order to ensure that the KHP sticking to the walls of the beaker is also placed into the volumetric flask. The 60 mL of distilled water is present in the volumetric flask, which is indicated by the fact that the water level in the flask is 3/4 of the way up the bulb of the volumetric flask. The flask was closed with the lid using a parafilm and inverted several times mixing the contents.

This was done two to three times until the KHP was completely dissolved. The parafilm was removed and the rinsed with a small amount of distilled water to make sure that the water drops into the volumetric flask. Distilled water was added enough to fill the flask with the etched of line. The flask was then again covered with the parafilm and inverted two to three times to mix the contents. The parafilm cover was removed, and was rinsed again to make sure all the solution remains in the volumetric flask. Water was added with a clean medicine dropper to bring the bottom of the meniscus to the etched line. The flask was then approved to the laboratory instructor and The volume of the water in the flask ended up being 100 mL, was recorded in the lab notebook. To calculate the molarity, find the number of moles of KHP must be calculated using the formula Moles = actual mass/ molar mass. Moles of KHP = 1.980 grams/ 208.252 molar mass of KHP

For the second part of the experiment, two burettes were used. After the two burettes were washed and the stopcock tested, the solutions were placed in their burettes. In the first burette, the KHP solution was filled to the brim and in the second burette, 100 mL of 0.10 M of NaOH stock solution contained the base NaOH. Both burettes were clamped in the vertical position and the initial volume was recorded. For both the NaOH and the KHP, the initial volume was 0 mL. Afterwards, the burette containing the standard KHP was placed approximately 25 mL of the KHP solution into an Erlenmeyer flask. The actual volume of KHP solution was therefore 25 mL and was added to the chart on the notebook. Then 25 mL of distilled water with 2 drops of phenolphthalein solution were added to the flask.

The Erlenmeyer flask was placed in a sheet of white paper under the burette containing the sodium hydroxide solution. The sodium hydroxide was added to the KHP solution drop by drop and was swirled gently and steadily in order to mix the acid and base solutions thoroughly. A first a transient pink color would appear as the sodium hydroxide was added but then quickly disappear in a few seconds later. As the titration continued, the pink color persisted for longer intervals but continued to disappear. Afterward, the sodium hydroxide was added in smaller portions. It was observed that the equivalence point was reached when there was a first permanent faint pink color in the KHP solution. However, the professor indicated that the equivalence point was overshot because the color of the solution was a dark pink therefore the KHP solution was aded to the solution until the pink disappeared. One drop of sodium hydroxide was was added to the solution but the dark pink color persisted and the process had to be done several times until a light pink color became permanent.

The final volume of acid burette reading is 50 mL. SInce the concentration of KHP was already calculated before, there was no need to calculate the concentration of KHP therefore the Molarity of KHP is 0.0951. The final volume of the base in the burette read 19 mL. To calculate the volume of the base required for Endpoint, the initial volume of the base was subtracted from the final volume of the base 19 mL – 0 mL = 19 mL . 19 mL of base was required for the endpoint. To calculate the concentration of the NaOH, it was noted that the moles of NaOH is equal to the moles of KHP. The certain equal was used to calculate the concentration. MNaOH x VNaOH = MKHP x VKHP

Therefore the concentration of the Base NaOH is .125 M. After this was calculated, it was shown and approved by the professor. All solutions were discarded in the appropriate waste container and all the materials were washed and dried. The difference between a stock solution and a standard solution is a standard solution is a solution of known concentration The stock solution, on the other hand, is a large volume of a common reagent. There is a distinction between the two because the stock solution is prepared using exacting techniques to ensure that the molarity is known to the high accuracy where as the stock solution is prepared to perhaps 1 or 2 significant figures of accuracy. The volume of the base needed to perform the KHP would have changed if the solution of 0.100 M barium hydroxide was used instead of sodium hydroxide solution depends on the concentration of the Sodium hydroxide solution.

For example if there was 0.100 M Sodium Hydroxide, the volume of the Barium Hydroxide needed is half the volume of Sodium hydroxide because one mole of Barium Hydroxide reacts with 2 moles of KHP, but for Sodium Hydroxide, one mole reacts with one mole of KHP. The average concentration of the sodium hydroxide solution for this experiment was .125 M. The sources of error that caused a variation in determinations of the concentration of sodium hydroxide maybe that not enough sodium hydroxide solution was used for titration since the volume of sodium hydroxide equals the volume of KHP, which is 25 mL. However for this experiment, the volume for sodium hydroxide was only 19 mL. Another error could be the significant digits in calculating for moles and the molarity.

CONCLUSION

The purpose of this experiment is to prepare a standard solution of potassium hydrogen phthalate and use titration to perform an acid/base reaction between the potassium hydrogen phthalate and sodium hydroxide to standardize approximately 0.10 M sodium hydroxide solution. The molarity of the KHP solution was 0.0951 M and the molarity of the NaOH is .125 M. The goal was reached when it required that 19 mL of Base was required for the endpoint, which is indicated by the very pale pink color. The sources of error were that the experiment was back titrated because the dark pink color indicated that the the equivalence point was over shot so in order to neutralize the solutions, the solution was back titrated. Another experimental error was at first only 9 mL of base was used to neutralize the acid, which would cause the molarity of the base to be higher than the molarity of the acid, which is incorrect since only one mole of NaOH is required to react with 1 mole of NaOH to in the acid/base reaction to produce water and Sodium Potassium phthalate, a salt.


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