Ka summary - Over lab for finding Ka PDF

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Over lab for finding Ka...

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Determination of Ka in a Weak Acid Introduction Strong acids can be completely dissociated in a dilute aqueous solution, while weak acids can only be partially dissociated in an aqueous solution. A weak acid in an aqueous solution will exist as a mixture of the molecular acid and an anion of the acid molecule. A way to explain this equilibrium reaction and the expression for an acid dissociation is below (1.). 1. HA ( aq ) + H 2O ( l ) ↔ H 3 O ( aq ) + A ( aq ) Ka=

[ H 3 O ][ A ] [ HA ]

In an equilibrium reaction, Ka can be determined experimentally by monitoring the pH while the weak acid is being titrated. At some point in the titration, the number of moles of base added is half the number of moles of acid originally present. This means 50% of the acid has been titration to Produce A and the other 50% remains as HA. This makes the ration A/HA. This point is referred to as the half equivalence point of the titration. Knowing the volume at the half equivalence point you can find the pH which we know is equal to pKa. As the titration is continued, the pH is always monitored when the base id being added and the pH at the half equivalence point will then give the Ka of the weak acid. In this experiment a titration was done with a sample of a weak acid solution and standardized with NaOH. After the completion of the titrations the concentration of the weak acid and the Ka for the weak acid could be found. In a lab setting, titration can be used to determine the unknown concentrations of a chemical or chemicals of interest in blood or urine. Results/Discussion This experiment had many sources of error. Not enough care was given when adding the NaOH when the titration was getting close to the equivalence point. The jump is very fast and was very easy to miss. Using an indicator that we were familiar with would have cut down on some error as well. One indicator began with a color and changed color less once the endpoint was reached. If we used an indicator we were used to we would have been able to tell when the end point was going to be reached easier. The concentration of NaOH that was previously calculated was then used to calculate the concentration of the acid for all three of the titrations. The mean concentration was .363 M and the standard deviation was .006 using the calculation below (2.). 2. M1V1 = M2V2 Using the half volume we found the half equivalence point for all three titrations. The pH was then calculated using the volume and the graphs and then calculated the Ka by using 10-pKa. The mean Ka is 1.60X10 -5 and the standard deviation was 2.8 X10 -5. Conclusion The Ka of a weak acid can be found experimentally through titration. The equivalence point can be found by monitoring the pH as the base is being titrated. The half equivalence

point can then be found. The pH can be concluded the Ka can be calculated by using the relationship that pH=pKa....