Domenica DiStasio April 24, 2019Exp. 5: Polyprotic Acids-- Determining Ka's Using pH Titration CurvesObjective: 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 of0.2. 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 AcidInitial buret reading = 11Initial pH of the citric acid = 2.74Burette Reading (mL)pH Reading Burette Reading (mL) pH Reading11 mL 2.74 9.9 mL 6.1315.2 mL 3.34 13.9 mL 6.5117.5 mL 3.65 14.9 mL 6.7520.2 mL 3.96 15.6 mL 7.1221.7 mL 3.44 17.4 mL 7.4323.6 mL 3.64 19.0 mL 9.0326 mL 3.86 20.2 mL 9.2828.8 mL 4.08 22.0 mL 9.5132.4 mL 4.31 24.7 mL 9.7134.6 mL 4.55 28.1 mL 9.9136.9 mL 4.74 32.5 mL 10.1238.2 mL 4.91 41.8 mL (refill to 1.5) 10.3740.9 mL 5.11 11.4 mL 10.6343.9 mL 5.31 31.6 mL 10.8348.1 mL (Refill to 3.3) 5.63 49.7 mL 10.926.2 mL 5.840 5 10 15 20 25 30 35024681012Citric AcidmL of NaOH addedpH~0.02M phosphoric acidInitial buret reading = 0Initial pH of the phosphoric acid = 2.4Volume of NaOH added.02 MpH Reading Volume of NaOH added.02 MpH Reading0 mL 2.40 .5 mL 8.1210.9 mL 2.67 .7 mL 8.512.1 mL 2.87 .7 mL 8.816.8 mL 4.74 .9 mL 9.07.3 mL 5.30 1.6 mL 9.341.5 mL 5.54 2.1mL 9.591 mL 5.78 3.3 mL 9.822 mL 5.98 4.2 mL 10.001.5 mL 6.21 7.4 mL 10.213.2 mL 6.40 17.3 mL 10.392.7 mL 6.62 16.8 mL 10.513.5 mL 6.82 <50 mL 10.681.1 mL 7.031.8 mL 7.30.7 mL 7.51.6 mL 7.880 20 40 60 80 100 120 140 160 180024681012Phosphoric AcidmL of NaOH addedpHCalculations: Phosphoric Acid:Ka1=10^ -2.67 = 2.14 x 10^-3Ka2=10^ -10.39 = 4.07 x 10^-11Ka3=10^ -10.51 = 3.09 x 10^-11Concentrations:M1V1 = M2V2 at the equivalence point(.02 M NaOH) (.0109L) = (M2) (.020 L)Phosphoric Acid: [Acid 1] = 0.0109 MM1V1 = M2V2 at the equivalence point(.02 M NaOH) (.0173L) = (M2) (.020 L)Phosphoric Acid: [Acid 2] = 0.0173 MM1V1 = M2V2 at the equivalence point(.02 M NaOH) (.0168L) = (M2) (.020 L)Phosphoric Acid: [Acid 3] = 0.0168 MCitric Acid:Ka1= 10^ -3.34 = 1.82 x 10^-3Ka2= 10^ -5.11 = 7.76 x 10^-6Ka3= N/AConcentrations:M1V1 = M2V2 at the equivalence point(.02 M NaOH) (.0042L) = (M2) (.020 L)Citric Acid: [Acid 1] = 0.0042 MM1V1 = M2V2 at the equivalence point(.02 M NaOH) (.0299 L) = (M2) (.020 L)Citric Acid: [Acid 2] = 0.0299 MDiscussion: 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 rightrate, 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 becauseif 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