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    Lab Report 10

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    Norayr Gulumain
    This experiment determined the dissociation constants and molecular weight of an unknown diprotic acid (malonic acid) through potentiometric titration with potassium hydroxide (KOH). The titration curve and its first derivative were used to find the equivalence points, from which the pKa values were calculated. The molecular weight was then determined using the moles of acid titrated at the first equivalence point. Sources of possible error in the experiment are also outlined.

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    Lab Report 10

    Uploaded by

    Norayr Gulumain
    4 views7 pages
    This experiment determined the dissociation constants and molecular weight of an unknown diprotic acid (malonic acid) through potentiometric titration with potassium hydroxide (KOH). The titration curve and its first derivative were used to find the equivalence points, from which the pKa values were calculated. The molecular weight was then determined using the moles of acid titrated at the first equivalence point. Sources of possible error in the experiment are also outlined.

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    analytical chemistry

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    Lab report 10

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    This experiment determined the dissociation constants and molecular weight of an unknown diprotic acid (malonic acid) through potentiometric titration with potassium hydroxide (KOH). The titration curve and its first derivative were used to find the equivalence points, from which the pKa values were calculated. The molecular weight was then determined using the moles of acid titrated at the first equivalence point. Sources of possible error in the experiment are also outlined.

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    4 views7 pages

    Lab Report 10

    Uploaded by

    Norayr Gulumain
    This experiment determined the dissociation constants and molecular weight of an unknown diprotic acid (malonic acid) through potentiometric titration with potassium hydroxide (KOH). The titration curve and its first derivative were used to find the equivalence points, from which the pKa values were calculated. The molecular weight was then determined using the moles of acid titrated at the first equivalence point. Sources of possible error in the experiment are also outlined.

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    of 7

    Norayr Kloumian

    Kevork boyadjian
    Ayya Yehia
    Chemistry 206 Lab
    Exp.10: PH Titration
    April 7th, 2022
    Rania Mansour
    I. Purposes:
     To learn the technique of potentiometric pH titration
     To compare different methods of determining equivalence points
     To determine the dissociation constants for an unknown polyprotic acid
    (malonic acid)
     To determine the molecular weight of the unknown acid

    II. Rxns:

     H 2 X + KOH ↔ KHX + H 2 O
     H X −¿+ KOH ↔ K X + H O ¿
    2 2

    III. Tables:

    Table 1: Preparation of diprotic acid (malonic acid H 2 X ¿

    Mass of Acid (± 0.0001 g ¿ Volume of acid (± 0.03 ml)

    1.0530g 250.0ml

    Table 2: Titration of malonic acid H 2 X with KOH

    V of KOH added in drops pH measurement

    0 2.13

    3 2.23

    6 2.36

    9 2.51

    12 2.65

    15 2.82

    18 2.97

    21 3.14
    24 3.29

    27 3.54

    30 3.95

    33 4.34

    36 4.70

    39 4.90

    42 5.11

    45 5.31

    48 5.44

    51 5.61

    54 5.78

    57 5.96

    60 6.20

    63 6.68

    66 11.26

    69 12.10

    72 12.42

    75 12.50

    78 12.67

    81 12.79
    Table 3: 1st Derivative table

    V of KOH in Drops ∆ pH
    ∆V

    3 0.0333

    6 0.0433

    9 0.0500

    12 0.0467

    15 0.0567

    18 0.0500

    21 0.0567

    24 0.0500

    27 0.0833

    30 0.137

    33 0.120

    36 0.130

    39 0.0667

    42 0.0700

    45 0.0667

    48 0.0433

    51 0.0567

    54 0.0567

    57 0.0600

    60 0.0800
    63 0.160

    66 1.53

    69 0.280

    72 0.107

    75 0.0267

    78 0.0567

    81 0.0400

    Table 4: Summary of Results:

    Equivalence point 1 Equivalence point 2

    pH V in drops pH V in drops

    Indicator 3.95 30 11.26 66

    Titration curve 3.90 30 9.40 65

    1st Derivative -- 30 -- 66

    Ka1 1.51 ×10


    −3

    Ka2 −6
    4.90 × 10

    IV. Calculations and Results:

     Visual method of determining the equivalnce point:


     Methyl orange changed the color from red to yellow
     Phenolphthalein changed the color from yellow to pink

    V 1 30
    = =15 drops p Ka1=2.82
    2 2
    3V 1 90
    = =45drops p Ka2=5.31
    2 2

     Calibration of a dropper:

    1ml of KOH 26 drops

     Calculation of the molar mass of the malonic acid H 2 X :

    1 ml of KOH →26 drops


    x → 30 drops
    30 ×1
    ml of KOH at the first equivalnce point=x= =1.15 ml
    26

    At the first equivalence point:

    n KOH n H X
    = 2

    1 1

    C KOH ×V KOH −3 −3
    nH X= =1.000 ×1.15 ×10 =1.15 ×10 moles (¿ 25.0 ml)
    2
    1

    −2
    n H X ( ¿ 250.0 ml ) =1.15× 10 moles
    2

    mH X mH X 1.0530
    nH X= 2
    → M H X= 2
    = =91.6 g /mol
    2
    MH 2 X
    2
    nH 2 X 1.15 ×10−2

    V. Possible Errors:

     Parallax errors
     Not fully immersing the pH electrode
     Uncalibrated balance (preparation of primary standard)
     Not protecting the primary standard KOH from CO 2 (which makes the concentration
    decreases because of acid-base rxn)
     Wrong number of added drops (excess)
     Uncalibrated pH meter
     Not using an indicator
     Errors in equivalence point detection

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