Acid Base Titration PDF

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lab acid base titration...

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Acid/Base Titration Purpose: To explore the relationship between different amounts of acid and base during titration by using the half-titration method.

Introduction: Titration is a method in which a solution of known concentration is used to assess the concentration of an unknown solution [1]. Usually, the titrant is applied to the known quantity of the analyte from the burette before the reaction is complete [1]. A series of neutralization reaction will occur until the end point. Knowing the amount of titrant applied makes it easier to determine the quantity of the unknown. Usually, an indicator is used to indicate the end of the reaction as it will permanently stay a colour [2]. The equivalency point is when there is enough titrant to neutralize the analyte. The experiment the equivalency point will be reached when the number of sodium hydroxide is equal to the number of moles of the unknown substance [1,2].

Diprotic acid is an acid that produces two H+ ions per atom. Sulfuric and carbonic acid are other examples of diprotic acids [3]. As shown by the following calculations, a diprotic acid dissociates in water in two stages [4]: 1. H2X (aq) ⇌ H+ (aq) + HX– (aq) 2. HX– (aq) ⇌ H+ (aq) + X2– (aq)

Procedure [4]: Part A: Unknown Acid Preparation 1. Measure out -0.120g of diprotic acid and record the mass and uncertainty

Vivian Xu 20228892 2. Dissolve 250mL of unknown acid in 100 mL of distilled water. Part B: pH vs Volume Plot 1. Connect the pH probe 2. Set up Logger pro to plot the pH curve Part C Burette Preparation 1. Fill a burette with 50mL of 0.1 NaOH, record the concentration. 2. Make sure the stopcock is closed and fill until past the 0.00mL and place burette on the clamp. 3. Drain some NaOH into a 400mL waste beaker which lowers the volume to less than 0.00mL. Make sure no bubbles appear. 4. Rinse off any reaming titrant 5. Place the stir bar in the beaker and place beaker on magnetic stirrer. Turn it on and slowly reach a steady mixing point. Make sure the probe is not hit by the magnetic stirrer and make sure the tip is able to record the ph. Part D: Titration 1. Using the Logger Pro to collect data click [collect] 2. Add the titrant in 1mL intervals after each 1mL press [keep] and type in volume added. The data points will then appear. 3. When titration is over click [stop]. 4. Click auto-scale to re-scale the graph. 5. Save the graph. Part E: Plotting the Second Derivative on Logger Pro

Vivian Xu 20228892 1. Find the most clearly defined equivalence point, this will be used to answer the questions. 2. Use the point to find the volume of NaOH used to reach the equivalence point. 3. Plot the second derivative and find when it interacts with zero 4. Take a screenshot of the second derivative plot. Include a copy of the pH curve and second derivative. Questions: 1. Concentration of NaOH: 0.1014 ± .0001 M First equivalence point: 15 mL ± 0.05 = 0.0150 L n = CV n = (0.1014 M) (0.0150L) n= 1.0647 x 10-3 mol

2. At the equivalence point there are 1.0647 x 10-3 mol as moles of acid = moles of base. 3. Mass of diprotic acid = 0.2341g ± 0.0002 - 0.118g ± 0.0002 !

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M =111.58 g/mol M = 112 g/mol

4. The acid is Maleic Acid.

Vivian Xu 20228892

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Percent Error = 3.45%

6. 1st equivalence point: 10.5 mL = 0.0105 L 1st half titration point: ½(0.0105) = 5.25x10-3 2nd equivalence point: 21 mL = 0.021 mL 2nd half titration point = = 0.0105 L+ 5.25x10-3 = 15.75x10-3

pH of 1st equivalence point pH = 2.9

pH of 2nd equivalence point pH = 6.2

The pka1 and pka2 are 1.94 and 6.22 and in comparison, to the values in the experiment which is 2.9 and 6.2 respectively [5].

Vivian Xu 20228892 7. Methyl orange was not a good choice of an indicator as it indicates the pH range of 3.14.4. A better option would have been Phenolphthalein as you would just need to titrate until it becomes colourless as the pH range is between 8.3-10. It appears as pink in basic solutions and clear in acidic solutions, so the endpoint is more recognizable and easy to see the change.

Data and Observations: Mass of weighing vial + acid: 0.2341 g ± 0.0002 Mass of Weighing vial empty: 0.1153 ± 0.0002 Concentration of NaOH: 0.1014. ± 0.0001 M Methyl Orange is used as the indicator Initial volume of burette 0.00mL Observation of Colour Change -

from deep orange to rust at pH: 4.96, volume of 7.49mL

-

from rust to peach at pH: 4.42, volume of 9.45mL

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from peach to yellow at pH: 5.85, volume of 14.12m

Graph of pH change over whole range

Vivian Xu 20228892

Graph of pH vs Volume of 0.1 M

pH at the change of colour Methyl Orange

3.96-4.42

Bromethymol Blue

6.87

Vivian Xu 20228892 Methyl Red

7.27

Phenolphthalein

7.72-9.5

References 1. Ball, D. and Key, J., 2021. Acid-Base Titrations. [online] Opentextbc.ca. Available at: [Accessed 5 February 2021]. 2. BBC Bitesize. 2021. Indicators - pH scale and indicators - GCSE Chemistry (Single Science) Revision - Other - BBC Bitesize. [online] Available at: [Accessed 5 February 2021]. 3. Chemed.chem.purdue.edu. 2021. Diprotic and Triprotic Acids and Bases. [online] Available at:

[Accessed 5 February 2021]. 4. Chem 112 Onq site, Documentation for Titration Lab, https://onq.queensu.ca/d2l/le/content/429885/viewContent/2955858/View 5. National Center for Biotechnology Information (2021). PubChem Compound Summary for CID 444266, Maleic acid. Retrieved February 5, 2021 from https://pubchem.ncbi.nlm.nih.gov/compound/Maleic-acid....