Determination of Amount of Acid…by Back Titration

Experiment 4

Experiment 4

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

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

?, n tabletdneutralize

HCl usedinitialHCl V[HCl]n ,

NaOHNaOHedHCl titrat VMn

titratedHClinitialHCl

tabletdneutralize

nn

n

,,

,

Acid-Base Definitions

Acids generate H+ in water H+ donors excess H+

Bases generate OH- in water H+ acceptors Excess OH-

Equilibrium in Water

14

253 101 o@

--weq ]][OH [H]][OHO [HKK

Small K equilibrium favors reactants

Review: H+ and OH-

[H+] 1 100 to 1 10-14 in water

[OH-] 1 10-14 to 1 100 in water

][H

K ][OH w-

14101 ]][OH[HK -w

][OH

K ][H

-w

Review: As [H+] rises, [OH-] falls

H+H+

H+

OH-OH-OH-

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14Acidic Neutral Basic

][H -pH log 0 to 14 in water

Relationships

H+ OH-

[H+] >[OH-]

Acidic soluti

on

Basic solutio

n

[H+] =[OH-] [H+] <[OH-]

Neutral

solution

H2O

Strong Acids (exp. 4)

100% dissociation / good H+ donor

equilibrium lies far to right

HCl H+ + Cl-

HCl H+ Cl-

HCl H+ Cl-

Rela

tive

#

mole

s

Before dissociation

After dissociation

Weak Acids (exp. 5)

<100% dissociation / not-as-good H+ donorequilibrium lies far to left

HCO3- H+ + CO3

2-

HCO3- H+

CO32-

Rela

tive

#

mole

s

Before dissociation

After dissociation

HCO3- H+

CO32-

Acid Dissociation Constant

Relative strength of acid

H A HA -

[HA]

]][A[H K

-

a

Strong acids: Ka is huge

Weak acids: Ka is small

Indicators are weak acids

HIn H+ + In-

Overview

0) Standardize NaOH with KHP

1) Find moles of acid, nH+

HCl(aq) H+(aq) + Cl-

(aq)

(H+ is H3O+)

HCl(aq) “stomac

h”

Overview

2) Neutralize some with antacid

Mg(OH)2 + 2 HCl

Mg2+ + 2Cl- + 2H2O

CaCO3 + 2 HCl

Ca2+ + 2Cl- + H2O + CO2

AntacidMg(OH)2 + CaCO3

HCl(a

q)

Overview

3) Back-titrate leftover HCl with NaOH

HCl + NaOH

Na+ + Cl- + H2O

NaOH titrant

Leftover HCl(aq)

Experimental Stages

used HClinitial V]HCl[n

nfinal

niniti

al HCl consumed by antacid

HCl consumed by titration0n final

n HClneutralized by tablet = n HClinitial ─ n HCltitrated

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]

@ eq. pt.: nNaOH = nKHP MM

KHP=204.23g/mol

Calculate: nNaOH nNaOH

3 trials Calculate average MNaOH MNaOH

Part 2 Standardization of HCl

Find HCl molarity Exactly 10mL HCl + drops BTB VHCl

Titrate with NaOH VNaOH

Given: [NaOH] MNaOH

@ eq. pt.: nHCl = nNaOH

Find: nNaOH used = [NaOH] . VNaOH

Calculate: nHCl titrated in 10 mLMHCl

Repeat Calculate average MHCl MNaOH

Example data

Use MHCl in part 3 (titration)

Standardization Part 1 NaOHTrial g KHP mL NaOH M NaOH

1 3.0000 29.4 0.49962 3.0000 29.1 0.50483 3.0000 30.9 0.4754

averages 3.0000 29.8 0.4929

Standardization Part 2 HClTrial mL HCl mL NaOH M HCl

1 10.00 20.5 1.01052 10.00 20.5 1.0105

averages 10.00 20.5 1.0105

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

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

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

Part 3 Acid Neutralized by Tablet

NaOH + HClremaining NaCl(aq) + H2O

Given: [NaOH], [HCl] MNaOH, MHCl

Find: nNaOH used VNaOH

@ eq. pt.: nNaOH = nHCl

Calculate: nHCl remaining nHCl

nHCl titrated = nNaOH used = [NaOH] . VNaOH

Example dataTitration Part 3 HCl

Trial g tablet mL HCl mL NaOH mol HCl titrated1 1.50 25.00 22.2 0.01432 1.51 25.00 21.4 0.01473 1.47 25.00 22.0 0.01444 1.50 25.00 21.5 0.0147

averages 1.50 25.00 21.8 0.0145avg mol neutralized/ g tablet 0.00972

Theoretical amount neutralized by tabletg base MM mol ratio mol HClneutralized

Mg(OH)2 0.11 58.33 2 0.0038CaCO3 0.55 100.08 2 0.0110

sum 0.0148%difference 1.58

Part 3 nHCl neutralized by tablet

HCl usedinitial V[HCl]n

nfinal

niniti

alHCl consumed by antacid

HCl consumed by titration

n HClneutralized by tablet = n HClinitial ─ n HCltitrated

NaOHNaOHedHCl titrat VMn

Part 3 Reporting ResultsAcid neutralized by tablet

110 mg Mg(OH)2 + 2 HCl MgCl2(aq) + H2O + CO2

550 mg CaCO3 + 2 HCl CaCl2(aq) + H2O + CO2

1.) Per mass

average,

2.) % acid neutralized relative to predicted

% of moles actually neutralizedvs. predictedstoichimetry

tablet

alexperiment HCl

g

n 3 good trials

%100n

n

al theoreticHCl

alexperiment HCl calculated

average

Report

Abstract

Results

Sample calculations including Part 1: individual [NaOH] + average & Part 2: individual [HCl] + average & Part 3: moles HCl initially

moles HCl titrated with NaOH

moles HCl neutralized by tablet

moles HCl per mass tablet

% predicted

Discussion/review questions

Strong acid-strong base titrations15_327

01.0

Vol NaOH added (mL)

50.0

7.0

13.0

pH

100.0

Equivalencepoint

At equivalence point:

mol H+ = mol OH-

nH+ present= nOH- added