Domenica DiStasio April 24 2019 Exp 5 Polyprotic Acids Determining Ka s Using pH Titration Curves Objective The objective of this lab was that we are trying to determine the Ka value using the pH titration curve that we derive from our experiment Procedure 1 Using a graduated cylinder place 20mL of the 0 02M citric acid into a small beaker If necessary add distilled water such that the tip of the pH probe is covered 2 Fill your buret with the 0 02M NaOH solution Record the exact molarity of this solution Record the initial buret reading Remember that this corresponds to 0 00mL of NaOH added Record the initial pH of the Citric acid 3 Carefully add the NaOH recording the volume of NaOH required to effect a pH change of 0 2 Continue this process until the pH reaches 12 4 Plot a graph of pH versus Volume of NaOH added and from this graph determine The Ka values for citric acid The exact concentration of the citric acid 5 Repeat steps one through five using the 0 02M phosphoric acid and determine the Ka values for phosphoric acid and the exact molarity of the phosphoric acid solution Data Collection 0 02M Cirtic Acid Initial buret reading 11 Initial pH of the citric acid 2 74 pH Reading Burette Reading mL pH Reading 11 mL 2 74 9 9 mL 6 13 15 2 mL 3 34 13 9 mL 6 51 17 5 mL 3 65 14 9 mL 6 75 Burette Reading mL 20 2 mL 3 96 15 6 mL 7 12 21 7 mL 3 44 17 4 mL 7 43 23 6 mL 3 64 19 0 mL 9 03 26 mL 3 86 20 2 mL 9 28 28 8 mL 4 08 22 0 mL 9 51 32 4 mL 4 31 24 7 mL 9 71 34 6 mL 4 55 28 1 mL 9 91 36 9 mL 4 74 32 5 mL 10 12 38 2 mL 4 91 41 8 mL refill to 1 5 10 37 40 9 mL 5 11 11 4 mL 10 63 43 9 mL 5 31 31 6 mL 10 83 48 1 mL Refill to 3 3 5 63 49 7 mL 10 92 6 2 mL 5 84 Citric Acid 12 10 pH 8 6 4 2 0 0 5 10 15 20 25 30 mL of NaOH added 0 02M phosphoric acid Initial buret reading 0 Initial pH of the phosphoric acid 2 4 Volume of NaOH added 02 M pH Reading Volume of NaOH added 02 M pH Reading 0 mL 2 40 5 mL 8 12 10 9 mL 2 67 7 mL 8 51 2 1 mL 2 87 7 mL 8 81 6 8 mL 4 74 9 mL 9 07 3 mL 5 30 1 6 mL 9 34 35 1 5 mL 5 54 2 1mL 9 59 1 mL 5 78 3 3 mL 9 82 2 mL 5 98 4 2 mL 10 00 1 5 mL 6 21 7 4 mL 10 21 3 2 mL 6 40 17 3 mL 10 39 2 7 mL 6 62 16 8 mL 10 51 3 5 mL 6 82 50 mL 10 68 1 1 mL 7 03 1 8 mL 7 30 7 mL 7 51 6 mL 7 88 Phosphoric Acid 12 10 pH 8 6 4 2 0 0 20 40 60 80 100 mL of NaOH added Calculations Phosphoric Acid Ka1 10 2 67 2 14 x 10 3 Ka2 10 10 39 4 07 x 10 11 Ka3 10 10 51 3 09 x 10 11 Concentrations M1V1 M2V2 at the equivalence point 120 140 160 180 02 M NaOH 0109L M2 020 L Phosphoric Acid Acid 1 0 0109 M M1V1 M2V2 at the equivalence point 02 M NaOH 0173L M2 020 L Phosphoric Acid Acid 2 0 0173 M M1V1 M2V2 at the equivalence point 02 M NaOH 0168L M2 020 L Phosphoric Acid Acid 3 0 0168 M Citric Acid Ka1 10 3 34 1 82 x 10 3 Ka2 10 5 11 7 76 x 10 6 Ka3 N A Concentrations M1V1 M2V2 at the equivalence point 02 M NaOH 0042L M2 020 L Citric Acid Acid 1 0 0042 M M1V1 M2V2 at the equivalence point 02 M NaOH 0299 L M2 020 L Citric Acid Acid 2 0 0299 M Discussion The difference between Ka of the given and my Ka is due to an experimental error It was very difficult to constantly raise the pH by 0 2 each time Even when we got the pH to raise at the right rate it was never the exact amount needed There was a lot of human error in this experiment Titrating the NaOH to increase the pH by only 2 at a time was difficult as even a single drop of NaOH would increase the pH by more than 2 It was really hard to do the citric acid one because if you added a drop the pH would chance drastically hence why we only did 2 citric acid Ka calculations Additionally the pH probes aren t that reliable either All of this error caused our graph to curve at much different rates than they were supposed to It was expected that they looked more exponential with plateaus in the middle The Ka values were definitely off and I think this was just all due to the human error that occurred during this lab