University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Determination of Ka for a weak Acdi
In the experiment preformed the objective is to titrate a weak acid with a strong base. In a titration of a weak acid with a strong base the titrant is the strong base and the analyte is a weak acid. The reaction that will occur is the direct transfer of protons from the weak acid to the hydroxide ion. The data gathered will be represented on the titration curve, a graph of the volume of titrant being the strong base plotted against the pH .The pH is an indicator of an acids strength. The titration curve can be used to determine the pKa. By reading the graph the equivalence point can be found; which is the point where equal parts acid and base have reacted by knowing this the half-equivalence can be found pH=pKa.
In the experiment pH paper will be used instead of a pH meter. The pH will be determined at the beginning and the end of the titration and the data table will be used to calculate the intermediate values. A burette is a more accurate piece of glassware used to deliver the titrate; in the lab being performed disposable pipet will be used making it very important to consistently dispense the same size drops. Before the titration the volume of a drop must be determined. A pipet is completely fill with distilled water. The average number of drops in a mL and the average quantity of a mL represented by on drop is calculated and recorded. Water is added drop by drop to a graduated cylinder from a pipet at the first, second and third mL lines the water drops are recorded. The average number of drops are calculated per mL. The average of the quantity of a mL represented by a drop is also recorded.
A data table is set up to represent the trail averages. First 2.0 mL of unknown acid is measured into graduated cylinder and then poured into a beaker the volume is the recorded. By using a toothpick a drop of acid is placed onto low portion of pH paper, the pH level is recorded. One drop of the phenolphthalein indictor is added to the acid and the color is recorded. The beaker is set on a white sheet of paper before moving on. Next, a well in the 24-well plate is filled with NaOH solution and then sucked up into an empty pipet. The pipet is the held vertically slowly adding drop by drop the NaOH into the beaker of the unknown solution. Drops are added until a color change occurs, changing to a faint pink for at least 30 seconds. A plastic spoon is used to stir after the addition of each drop. The number of drops of NaOH is recorded and the equivalence point is now determined. A drop of the acid is now transferred by toothpick to the high range pH indicator strip. The pH level of the acid is recorded before titration. The contents of the beaker are poured down the drain and all equipment is thoroughly cleaned. The above procedure is repeated twice more, all data is recorded to 4 decimal places for each trail on the data table. The average of the 3 trails is calculated and data is recorded.
At the top of the pH column the unknown’s acid’s starting pH level before titration is entered. Next, the pH level of the acid after the titration, at its equivalence point is entered at the bottom of the pH column. The appropriate pH levels for each 2 drop interval is then calculated, by subtracting the initial pH from the final pH and dividing the resulting difference by the number of rows minus 1. This number is then added to the previous pH value.
A graph is then made, pH is plotted on the y axis and volume of NaOH added on the x axis. This represents the titration curve. The pH that corresponds with the equivalence point and half equivalence points are located and the pKa is determined for the unknown acid, pH=pKa. The Ka is determined by taking the inverse log of the pKa(10-pka).