Determination of Amount of Acidby Back Titration

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Determination of Amount of Acidby Back Titration

• Experiment 7

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Experiment 7

• Goal
• Determine amount of acid that can be neutralized
  by a commercial antacid
• Method
• React antacid tablet with excess stomach acid
  (HCl)
• Perform a back titration using standardized NaOH
  solution

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Big Picture

• 1) Determine amount of acid (HCl) in flask
• 2) Neutralize some HClwith antacid tablet
  (CaCO3,Mg(OH)2)
• 3) Titrate remaining acid with base (NaOH)
• 4) Find amount of acid neutralized by tablet

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Acid-Base Definitions

• Acids
• generate H in water
• H donors
• excess H
• Bases
• generate OH- in water
• H acceptors
• Excess OH-

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Equilibrium in Water
Small K ? equilibrium favors reactants
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Review H and OH-
H 1 ? 100 to 1 ? 10-14 in
water OH- 1 ? 10-14 to 1 ? 100 in water
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Review As H rises, OH- falls
0 to 14 in water
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Relationships
H
OH-
H2O
H gtOH-
H OH-
H ltOH-
Basic solution
Acidic solution
Neutral solution
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Strong Acids (exp. 7)

• 100 dissociation / good H donor
• equilibrium lies far to right
• HCl ? H Cl-

Before dissociation
After dissociation
Relative moles
HCl H Cl-
HCl H Cl-
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Weak Acids (exp. 13)

• lt100 dissociation / not-as-good H donor
• equilibrium lies far to left
• HCO3- H CO32-

Before dissociation
After dissociation
Relative moles
HCO3- H CO32-
HCO3- H CO32-
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Acid Dissociation Constant

• Relative strength of acid

Strong acids Ka is huge Weak acids Ka is small
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Indicators are weak acids

• HIn H In-

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Overview Exp. 7
HCl(aq) stomach

• 0) Standardize NaOH with KHP
• 1) Find moles of acid, nH
• HCl(aq) ? H(aq) Cl-(aq)
• (H is H3O)

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Overview Exp. 7

• 2) Neutralize some with antacid
• Mg(OH)2 2 HCl ?
• Mg2 2Cl- 2H2O
• CaCO3 2 HCl ?
• Ca2 2Cl- H2O CO2

Antacid Mg(OH)2 CaCO3
HCl(aq)
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Overview Exp. 7

• 3) Back-titrate leftover HCl with NaOH
• HCl NaOH ?
• Na Cl- H2O

NaOH titrant
Leftover HCl(aq)
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Experimental Stages
n HClneutralized by tablet n HClinitial - n
HCltitrated
ninitial
HCl consumed by antacid
HCl consumed by titration
nfinal
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Part 1 Standardization of NaOH

• Find molarity of NaOH
• g KHP(calculated in prelab) mKHP
• in 50 mL H2O drops BTB
• Titrate with NaOH VNaOH
• Given 0.5 M NaOH
• _at_ eq. pt. nNaOH nKHP MM
  KHP204.23g/mol
• Calculate nNaOH nNaOH
• 3 trials
• Calculate average MNaOH ?M?NaOH

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Part 2 Standardization of HCl

• Find HCl molarity
• Exactly 10mL HCl drops BTB VHCl
• Titrate with NaOH VNaOH
• Given NaOH MNaOH
• _at_ eq. pt. nHCl nNaOH
• Find nNaOH used NaOH . VNaOH
• Calculate nHCl titrated in 10 mL MHCl
• Repeat
• Calculate average MHCl ?M?NaOH

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Example data
Use MNaOH in parts 2 (HCl standardization) and 3
(titration)

• Use MHCl in part 3 (titration)

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Part 3

• Antacid tablet reaction
• Rinse all glassware
• Record mass of 4 antacid tablets
• Label 4 125 mL Erlenmeyer flasks
• Add exactly 25 mL HCl and 1 tablet to each
• Gently boil 5 minutes
• Cool and add 2 drops BTB indicator
• First trial guide for 3 more
• Titrate with NaOH to equivalence point
• Save as standard color
• Repeat more 3 times

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Why is heating necessary?

• Heat removes CO2 made in HCl/antacid reaction
• CaCO3 2 HCl CaCl2(aq) H2O CO2
• CO2 H2O H2CO3(aq) HCO3-
  H
• Removing CO2 allows maximum amount of acid to be
  neutralized

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Part 3 Acid Neutralized by Tablet

• NaOH HClremaining ? NaCl(aq) H2O
• Given NaOH, HCl MNaOH, MHCl
• Find nNaOH used VNaOH
• _at_ eq. pt. nNaOH nHCl
• Calculate nHCl remaining nHCl
• nHCl titrated nNaOH used NaOH . VNaOH

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Example data
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Part 3 nHCl neutralized by tablet
n HClneutralized by tablet n HClinitial - n
HCltitrated
ninitial
HCl consumed by antacid
HCl consumed by titration
nfinal
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Part 3 Reporting Results

• Acid neutralized by tablet
• 110 mg Mg(OH)2 2 HCl CaCl2(aq)
  H2O CO2
• 550 mg CaCO3 2 HCl CaCl2(aq)
  H2O CO2
• 1.) Per mass
• average, s
• 2.) acid neutralized relative to predicted
• of moles
• actually neutralized vs.
  predictedstoichimetry

3 good trials
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Report

• Abstract
• Results
• Sample calculations including
• Part 1 individual NaOH average s
• Part 2 individual HCl average s
• Part 3 moles HCl initially
• moles HCl titrated with NaOH
• moles HCl neutralized by tablet
• moles HCl per mass tablet
• predicted
• Discussion/review questions

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Strong acid-strong base titrations

• At equivalence point
• mol H mol OH-
• nH present nOH- added