Acid Base Titration PDF

Preview

Summary

Assignment on acid base titration...

Description

1

Experiment 9: (2 weeks) (ABT) Acid/base Titration Purpose The purpose of the first part of the lab is to standardize a solution of 1M NaOH with the help of a primary standard, potassium hydrogen phthalate. On the other hand, in the second part, the dissociation constants of an unknown diprotic acid is to be determined via the half-titration method.

Introduction1 Titrations are common methods used for determining the unknown concentration of a solution. They involve the addition of a standard solution of known concentration in order to alter the pH in such a way so that the equivalence point, the point where the moles of base added equals the moles of acid, na=nb can be reached. The number of moles can be found from the multiplication of the volume of a solution with its concentration n=VC. When sufficient titrant has been added to reach the equivalence point, the following equation can be used to find the concentration of the unknown solution: n a= VaCa=VbCb=nb

[Equation35]

Moreover, the point at which a solution has reached its equivalence point can be detected via electrochemical monitoring or with the help of an appropriate chemical indicator. An indicator is basically a weak organic acid whose colour is different from that of its conjugate base. When the indicator has equal concentrations of conjugate acid and base, the conjugates colours will be equal in intensity. This point is called the endpoint as opposed to the equivalence point since the two do not coincide perfectly. In the first part, the unknown concentration of a NaOH solution is to be found with the help of potassium hydrogen phthalate (KHP) dissolved in water as the reference material. The dissolved KHP dissociates into K+ ion and HP- ion [Equation 36] which is in turn titrated with the OH - ion from the (0.1M) NaOH using phenolphthalein as an indicator and as a result, producing P 2- ion and water [Equation 37]. The end of this titration is demonstrated by the clear solution turning light pink at the endpoint. Lastly, the concentration of the NaOH solution is determined using the amount of KHP used and the volume of NaOH required in the titration. Diprotic acids such as sulfuric acid and carbonic acid donates two H + ions per molecule, have two Ka values and two equivalence points. At the first equivalence point, all H + ions have combined with all OHions. Hence, the volume of titrant needed to neutralize the first hydrogen atom from every molecule in the diprotic acid is equal to the titrant volume required to neutralize the second. As a result of this, only the first equivalency is needed to be found to determine the acid’s unknown concentration. Lastly, the acid’s concentration can be used together with its mass to calculate the molar mass of the diprotic acid which in turn used to find its identity. The dissociation constants of the diprotic acid can be determined via the half-titration method.

2

|

Procedure2 Part I: Monoprotic Acid A 50mL burette was filled with the 0.1M NaOH solution up to the 0.00mL mark. Next, about 0.8g of KHP was measured directly into a 250mL Erlenmeyer flask and 50mL of RO water was poured in order to dissolve it. 2 drops of phenolphthalein were then added to it. The flask was put on the magnetic stirrer with a stir bar in it to mix the contents while the KHP was titrated with the NaOH being added drop wise from the burette. The above procedure was done three times more. Part II: Diprotic Acid The final amount of the unknown diprotic acid was mixed with 100mL of RO water in a 250mL beaker in which a few drops of methyl red was then added. Afterwards, a pH probe was connected with the Logger Pro in order to produce a graph of pH vs. Volume. On the other hand, a 50mL burette was filled with approximately 1.0M NaOH solution. The beaker with a magnetic stir bar in it was placed on the hot plate and the pH probe was then submerged securely in the unknown solution. For the whole time of titration, data was recorded after 1mL NaOH was added every time. Lastly, the data from the plot was used to form the second derivative curve and the point at which it crosses zero determined.

Observations In the first part of the experiment, the white powder KHP dissolved into a colourless solution. Next, the colour of the colourless solution turned to light pink at the endpoint due to the indicator phenolphthalein having no colour in low pH and light pink colour in high pH. On the other hand, in the second part of the experiment, the colour of the solution changed from red to yellow at the endpoint due to the indicator methyl red having red colour in low pH and yellow in high pH. Questions Questions for Part I 1. What is the concentration of the base from the stock bottle? Show any calculations and uncertainty calculations.

Trail 1

Trial 2

Trial 3

Mass of Vial

0.6785 g

0.6766 g

0.6696 g

Mass of Vial+ KHP

1.4925 g

1.5106 g

1.4700 g

Mass of KHP

0.8140 g

0.8340 g

0.8004 g

Moles of KHP

0.00399 mol

0.00408 mol

0.00392 mol

Initial Burette Volume

0.00 mL

0.00 mL

0.00 mL

3

Final Burette Volume

38.80 mL

39.50 mL

38.90 mL

Volume Used

38.80 mL

39.50 mL

38.90 mL

NaOH Concentration

(0.00399/38.80 x 10-3)= 0.103 mol/L

(0.00408/39.50 x 10-3) = 0.103 mol/L

(0.00392/38.90 x 10-3) = 0.101 mol/L

Average NaOH Concentration=(0.103 + 0.103 + 0.101)/3 = 0.102 mol/L Uncertainty needs to be within 1%: |0.103 - 0.101|/0.102 = 0.01

Questions for Part II 2. Calculate the molecular weight of the diprotic acid in g/mol using the mass of diprotic acid that you measured in the first step of the procedure and the moles you determined from the titration results (Show all your work). Mass of Diprotic Acid= 0.1202 g Concentration of NaOH used = 0.1033 M Volume required to neutralize the acid = 18.76 mL Moles of NaOH = (0.1033 x 18.76 x 10-3) = 1.938 x 10-3 mol Moles of Diprotic Acid = ( 1.938 x 10-3/2) = 9.69 x 10-4 mol Molecular weight of Diprotic Acid = (0.1202/9.69 x 10-4) = 124 g/mol 3. From the following list of five diprotic acids, identify the unknown diprotic acid that you used. Note that not everyone uses the same acid. Diprotic Acid Oxalic Acid Malonic Acid

Formula H2C2O4·H2O H2C3H2O4

MW 126 104

Maleic Acid Malic Acid

H2C4H2O4 H2C4H4O5

116 134

Tartaric Acid

H2C4H4O6

150

The diprotic acid is oxalic acid. 4. Determine the percent error between the molecular weight value you calculated in step 2 and that of your chosen acid. Percent error = (126-124/126) x 100 = 2% 5. Determine the pH for the first half-titration point (½ of the first equivalence point volume) and the second half-titration point (halfway between the first and second equivalence points). Compare these values to the pKa1 and pKa2 values for your chosen acid (reference).

4

First half-titration volume= 18.76/2 = 9.38 mL pH= 3.57 pKa1= 3.57 Second half-titration volume= 28.5/2 = 14.25 mL pH= 4.50 pKa2= 4.50 The literature values for pKa1 and pKa2 are 1.25 and 4.27 respectively. The experimental values, pKa1=3.57 and pKa2= 4.50 slightly differ from the literature values due to some systematic errors such as adding a few more drops to the analyte mistakenly and missing the endpoint, hence raising its pH.

6. By comparing various indicator results with other groups around you, which indicator(s) do you think worked for this experiment, i.e., the colour changed very near the equivalence point. The indicator used was methyl red. The pH range was 8.05-10.42 and the equivalence point was 18.76mL. Data from Alyssa Green(20158921): Indicator: Bromothymol -blue pH=9.73 Equivalence point= 20.89 mL Data from Sophie Dembski(20159875) and Mya Gillberry(20155479): Indicator: 7.98-10.46 pH= 7.98-10.46 Equivalence point= 20mL Based on these results, phenolphthalein worked best for this experiment.

5

6

References 1. Wu, Gang. Queen’s Chemistry First-Year laboratory Manual Chemistry 112; Queen’s University: Kingston ON, 2019-2020; p.109-121 2. Carey Bissonnette, F. Geoffrey Herring, Jeffry D. Madura, Ralph H. Petrucci - General Chemistry. Principles and Modern Applications (2016, Pearson) 3. https://www. studoc.com – Acid Base Titration...