Acid Content of Beverages A titration exercise SUSB - 010
2
Slide 4
characterize theacidity How can we characterize the acidity of
a natural mixture of unknown acids? strongweak What do chemists
mean when they talk about acids being strong or weak? ? QUESTIONS ?
quantitativemeasure What is a quantitative measure of the strength
of an acid? titrations How do we conduct titrations and what can we
learn from them? 3
Slide 5
Concepts: Strong/Weak Acids Acid Dissociation / K a
Concentration Titration Titration curve Equivalence point End point
Indicator Mole Relationships Polyprotic acids Total available acid
pH & pK a Logarithms Techniques: Apparatus: BuretpH Meter
Graduated Cylinder Titration 4 pH Measurement
Slide 6
MOLES, LITERS & CONCENTRATION UNITS mL mgsmall numbers of
grams. 1 kg1 mole1 Liter In lab, we generally measure volumes in mL
& weights in mg or small numbers of grams. We dont normally use
even close to 1 kg, 1 mole or 1 Liter of anything. 0 To avoid
writing quantities with zer0s after decimal point, procedures and
data sheets often specify: volumes in m mL (= 1 / 1000 liter) and
molar quantitites in m mmol (= 1 / 1000 mol) & weights in m mg
(= 1 / 1000 g) 0.03421 L 00 0.002845 mol m 34.21 mL m 2.845 mmol 0
0.0757 g m 75.7 mg 5
Slide 7
E.g., 6 M HCl has6 mmol of HCl in 1 mL, also 1.0 m mol of NaOH
weighs 40 m g Similarly, for atomic and molecular weights: atomic
weight of carbon is 12 m g / m mol molar mass of vanillin is 152
mg/mmol same numerical value For molar concentrations, M (molarity)
has same numerical value in mol / L and in mmol / mL mm HCl 6.0 M 6
mol mol 1000 mmol Y = Y = Y L L mL 1000
Slide 8
Background Strong and Weak Acids (and bases) Acids and bases
can be characterized by the extent to which they dissociate in
solution STRONG Fully Dissociated E.g., in water, HCl dissolves to
give a STRONG acid HCl (aq) H + (aq) + Cl - (aq) WEAK Partially
Dissociated 7 HCl (g) + H 2 O (l) proceeds virtually to completion
(no undissociated HCl )
Slide 9
Acetic Acid ( CH 3 COOH), dissolves in water to give a WEAK
acid CH 3 COOH (aq) CH 3 COO - (aq) + H + (aq) Almost all CH 3 COOH
exists in ASSOCIATED form Associated proceeds only slightly in
forward direction. Dissociated 8 group defines an organic acid
The
Slide 10
Analogously, NaOH dissolves in water to give a STRONG base:
NaOH (aq) Na + (aq) + OH - (aq) 9 NaOH (s) + H 2 O (l) proceeds
virtually to completion (no undissociated NaOH ), dissolves in
water to give a WEAK base, NH 3 (aq) NH 4 + (aq) + OH - (aq) NH 3
(g) + H 2 O (l) whereas, ammonia (NH 3 ) proceeds only slightly in
forward direction.
Slide 11
A QUANTITATIVE measure of strength or weakness of an acid ( or
base ) is its DISSOCIATION CONSTANT, K a For reaction HA H + + A -
acid dissociation constant, K a, is defined as [ H + ] [ A - ] K a
= ------------- [ HA ] LARGE K a ( >> 1) STRONG ACID SMALL K
a ( 9 pH < 9 38">
PHENOLPTHALEIN Use PHENOLPTHALEIN, which is itself, a weak
DI-PROTIC ACID. ColorlessPink PHENOLPHTHALEIN Color change for
PHENOLPHTHALEIN occurs when pH increases from 9. pH > 9 pH <
9 38
Slide 40
Before end point At end point Past end point 39 If you bring in
your own beverage, remember that you will need to be able to tell
when the color changes from its original color to pink.
Slide 41
1.) Determine TOTAL AMOUNT OF ACID in measured sample of
beverage.HOW? by titrating the sample with NaOH of known
concentration. NaOH reacts with ALL OF THE ACID*, dissociated, or
not. Will not detect any acids with pK a s > 9 (pH at which
phenolphthalein signals end point) We make two measurements: This
tells us TOTAL DISSOCIATED + ASSOCIATED ACID in the beverage.
40
Slide 42
2. Measure pH of beverage to determine H + concentration of
beverage using a pH METER This tells us CONCENTRATION OF
DISSOCIATED H + in beverage, [ H + ] 41 Electronic device designed
to measure hydrogen ion concentration in aqueous solutions
Slide 43
PROCEDURE Beverages you bring in vary widely in acid content.
Cannot predict how much of it to use. In such instances, we always
do a: To get reasonable precision from buret readings, always try
to use net volumes between 20 mL 30 mL in a titration. PRELIMINARY
TITRATION Objective: To determine how much of the beverage we need
to use to consume the desired amount (20 30 mL) of NaOH. 42
Slide 44
If 30.0 mL requires 15.45 mL of base X39 X = 20 X 30.0 / 15.45
= 39 3958 So, if we use between 39 and 58 mL of our beverage, we
will assure that the amount of NaOH required will be in the correct
range (20-30 mL) X 30.0 X -------- = ----- 15.45 20 Y 30.0 Y
-------- = ----- 15.45 30 X X mL requires 20 mL of base Y Y mL will
require 30 mL Y58 Y = 30 X 30.0 / 15.45 = 58 43
Slide 45
Example: 20.0 mL of a beverage requires 10.0 mL of NaOH to
reach the phenolphthalein end point. How much beverage will use
30.0 mL of the NaOH solution? 44 A.20 mL B.30 mL C.60 mL
Slide 46
Example: How much beverage 20.0 mL of a beverage requires 10.0
mL of NaOH to reach the phenolphthalein end point. How much
beverage will use 30.0 mL of the NaOH solution? C C60 mL 45 X mL
beverage X mL beverage 20.0 mL beverage = 30.0 mL NaOH 10.0 mL NaOH
20.0 mL beverage X mL beverage X mL beverage = 30.0 mL NaOH 10.0 mL
NaOH
Slide 47
one Having determined how much juice you need to conduct one
titration, you can compute how much beverage you will need to
complete the exercise. You are asked to do no more than 4
titrations (in addition to the preliminary titration) and report
the best 3 out of those 4. So, if x mL is required to do one
titration, you need, at most, (4x + 20) mL of beverage. If you do
not have enough beverage to complete exercise, use juice provided
in laboratory. (But you must do another preliminary titration.)
Based on the preliminary titration, Calculate this quantity Based
on the preliminary titration, Calculate this quantity. If you
brought sufficient beverage, continue using your beverage. 46
Slide 48
For simplicity, assume beverage contains single weak monoprotic
acid HA H + + A - with acid dissociation constant K a For this, we
need pH of untitrated (and undiluted) beverage. I.e., [ H + ] = 10
-2.7 = 2.0 X 10 -3 All that remains it to calculate (effective) K a
for acid(s) in beverage. [ H + ] [ A - ] K a = --------- [ HA ] (
2.0 X 10 -3 ) 2 = ----------------- 0.124 = 3.2 X 10 -5 Suppose
result of titration is Total Acid = 0.124 M & measured pH = 2.7
47 From pH Total Acid [ H + ] = [ A - ] Should subtract 2.0 X 10 -3
But to 2 sig figs, can ignore
Slide 49
REMEMBER: Second QUIZ will be given at the beginning of the
acid in beverages lab period. (15 minutes) It will cover:
SUSB-030,SUSB-004,SUSB-039 48
Slide 50
Slide 51
Stamp out Chemophobia U CHEMISTRY TELL EM This message is
approved by the USB Department of Chemistry
Slide 52
NEXT EXERCISE Strength of Vinegar by Acid Base Titration Read
SUSB-011 and Do Prelab A TEST EXERCISE 105 Points 51