Acid / Base

Titration

Professor Bob Kaplan University Department of Science

The most distinctive feature of an acidic or

basic compound is ionic dissociation.

Compounds dissociate into their

individual cations and anions.

Acids H+ cations

Bases OH- anions

Strength of an acidic or basic compound is

determined by the degree of dissociation.

Less stable compound

More ionic dissociation More

charged ions in solution

Stronger acid or base

HCL H+ + Cl-

Strong acid driven to products.

Products produced includes: H+ cations

More products ( more H+ ) Stronger acid

Strong acids dissociate completely

to form products.

pH indicates degree of dissociation. 

pH measures the concentration of H+ ions.

ph [ H+ ]

pOH [ OH- ]

ph = - log [ H + ]

[ H + ] = 0.1 M = 10 -1 M

log 10 -1 = - 1- l

og = 1

[ H + ] = 0.000001 M = 10 -6 M

log 10 -6 = - 6- log = 6

Lower pH Higher Acidity

Strong acids dissociate to products

HCL H+ + Cl-

Measure strengths by Dissociation Constant

K =

Larger K Higher [ H + ]

Smaller K Lower [ H+ ]

 

Stronger acid Higher [ H + ] 

Larger K Stronger acid

NaOH Na+ + OH-

K =

Larger K Higher [ OH - ]

Larger K Stronger base

Sulfuric AcidH2SO4 --> 2

H+ + SO42 protons

liberated per molecule.

Phosphoric acid (in soda pop)

H3PO4 --> 3 H+ + PO4

3 protons liberated per molecule !

 

Acids produce H+ cations

HCl = H+ + Cl-

Each mole of HCL produces 1 mole of H+

Bases produce OH- anions NaOH = Na+ + OH- Each mole of NaOH produces I mole OH-

H+ + OH- = H2O

pH 7 = neutral point

ph 7: [ H+ ] = [ OH- ] = 10 -7

Ionic concentrations are equal.

Investigate the strength of 5 different

commercial antacids by chemical titration.

A titration is a controlled way of measuring the

amount of acid it takes to neutralize the base. 

If you know how much base is used,

then you can tell how much acid is present.

Using HCl and NaOH as our acid and base,

All mole ratios are 1:1.

In the chemistry laboratory, we use volumetric

measurement for solutions. Examples include:

Beakers, flasks, burettes, pipettes,

eye droppers

Typically in liters (L) or milliliters (mL)

Control the volume of acid or base using a stopcock

1-way valve at the base of a calibrated buret in order

to deliver the solution of acid or base drop-by-drop.

Measurement of the concentration of solution.

M = Molarity

Molar concentration = moles / liter = M

Measure volume in liters (or milliliters ml)

Vol ( L ) x Conc. ( mol / L ) =

Number of Moles

Our objective is to determine how much base is in

each antacid tablet. To determine this indirectly:

Neutralize the base with an acid.

Since we have a 1 : 1 ratio of acid : base

Number of moles of acid added

= Number of moles base in antacid tablet

Begin by adding extra acid.

Add 40 mL of 0.1 molar HCl to each one

How many moles of acid is that ?

 ( 40 mL ) * ( 1 liter / 1000 mL ) = 0.040 liters

Vol. x Conc. = Moles 

( 0.040 liters ) ( 0.1 moles / liter ) = 0.004 moles

View this 0.004 moles HCl as stomach acid.

That is the total amount of stomach acid after

dinner tonight. Then eat one tablets

But one

tablet is not enough.

• So in order to neutralize all of the acid,

Add MORE base.

The strongest tablet (base) will require

the least addition of more base.

The strongest tablet will already have the most

base and will thus require the least addition of

more base (NaOH).

That is the key to the first question.

Thus, there are no calculations necessary.

It is purely qualitative.

Compare volumes of added NaOH

Not all tablets weigh the same amount.

Break it down to a "per gram" basis

Change the volume added to:

Moles per gram

Moles added / gram dosage

 

Maalox requires 24.1 mL NaOH

NaOH Concentration = [ NaOH ] = 0.05 M

Molarity = M = moles / liter

( 0.024 L ) x ( 0.05 moles / liter)

= 1.21 x 10 -3 Moles

Consider the weight of each tablet in grams

Maalox: 20.0 grams

Want moles / gram

Divide the number of moles by

the weight of that tablet in grams

[ 1.21 x 10 -3 moles ] / 20 grams

= 6.05 x 10 -5 mol / g

•So which is the strongest antacid tablet on a per gram basis ?

•Which tablet requires the least amount of acid to neutralize the base in the tablet …..

…..in moles / gram ?

See the following websites:

http://en.wikipedia.org/wiki/Acid-base_titration

 http://en.wikipedia.org/wiki/PH_indicator

H2SO4 is a diprotic acid.

It releases 2 protons (H+) per acid molecule.

 H2SO4 = 2 H+ + SO4 (2-)

Thus, it is

likely to be at least twice as strong. Thus, at

the same concentration (0.1 M), we need

to add only half as much acid. 

Balance the equation:

Al (OH)3 + HCl AlCl3 + H2O

*Hint:

There are 3 hydrogen atoms on the left,

and only 1 H atom on the right

3 H2O needed on the right.