1
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
2
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-
3
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=
+
4
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 14
Acidic Neutral Basic
][H -pH +
= log 0 to 14 in water
5
Relationships
H+ OH-
[H+] >[OH-]
Acidic
solution
Basic
solution
[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
# m
ole
s
Before
dissociation
After
dissociation
6
Weak Acids (exp. 5)
<100% dissociation / not-as-good H+ donor
equilibrium lies far to left
HCO3- H+ + CO3
2-
HCO3- H+ CO3
2-
Rela
tive
# m
ole
s
Before
dissociation
After
dissociation
HCO3- H+ CO3
2-
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
7
Indicators are weak acids
HIn H+ + In-
8
Overview
0) Standardize NaOH with KHP
1) Find moles of acid, nH+
HCl(aq) → H+(aq) + Cl-
(aq)
(H+ is H3O+)
HCl(aq)“stomach”
Overview
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)
9
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
ninitial
HCl consumed
by antacid
HCl consumed
by titration0n final =
n HClneutralized by tablet = n HClinitial ─ n HCltitrated
10
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 ⟨M⟩NaOH
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 mL MHCl
� Repeat
� Calculate average MHCl ⟨M⟩NaOH
11
Example data
Use MHCl in part 3 (titration)
Standardization Part 1 NaOH
Trial g KHP mL NaOH M NaOH
1 3.0000 29.4 0.4996
2 3.0000 29.1 0.5048
3 3.0000 30.9 0.4754
averages 3.0000 29.8 0.4929
Standardization Part 2 HCl
Trial mL HCl mL NaOH M HCl
1 10.00 20.5 1.0105
2 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
12
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
13
Example dataTitration Part 3 HCl
Trial g tablet mL HCl mL NaOH mol HCl titrated1 1.50 25.00 22.2 0.0143
2 1.51 25.00 21.4 0.0147
3 1.47 25.00 22.0 0.0144
4 1.50 25.00 21.5 0.0147
averages 1.50 25.00 21.8 0.0145
avg mol neutralized/g tablet 0.00972
Theoretical amount neutralized by tablet
g base MM mol ratio mol HClneutralizedMg(OH)2 0.11 58.33 2 0.0038
CaCO3 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
ninitial
HCl consumed
by antacid
HCl consumed
by titration
n HClneutralized by tablet = n HClinitial ─ n HCltitrated
NaOHNaOHedHCl titrat VMn ⋅=
14
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, σ
2.) % acid neutralized relative to predicted
% of moles
actually neutralized
vs. predictedstoichimetry
tablet
alexperiment HCl
g
n3 good trials
%100n
n
al theoreticHCl
alexperiment HCl×
calculated
average
Report
Abstract
Results
Sample calculations including
� Part 1: individual [NaOH] + average & σ
� Part 2: individual [HCl] + average & σ
� Part 3: moles HCl initially
moles HCl titrated with NaOH
moles HCl neutralized by tablet
moles HCl per mass tablet
% predicted
Discussion/review questions
15
Strong acid-strong base titrations
0
1.0
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