Illustrated Guide to Home Chemistry Experiments

Chapter 11 Laboratory: Acid-Base Chemistry 193

LABORATORY 11.1:

dETERmINE THE EffECT of CoNCENTRATIoN oN pH

pH is a metric used to specify the acidity (or

basicity, also called alkalinity) of an aqueous

solution. The pH of a solution is determined

by the relative activities of the hydronium

(H 3 O+) ions and the hydroxide (OH–) ions in

the solution. The pH of a solution in which the

activities of these two ions are equal—such as

pure water at 25°C—is 7.00. A solution in which

the activity of the hydronium ions is higher than

the activity of the hydroxide ions has a pH lower

than 7 and is acidic. A solution in which the

activity of the hydroxide ions is higher than the

activity of the hydronium ions has a pH greater

than 7, and is basic.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ beaker, 150 mL (6)

£ volumetric flask, 100 mL

£ pipette, 10 mL

£ pH meter

£ hydrochloric acid, 1 m (100 mL)

£ sulfuric acid, 1 m (100 mL)

£ acetic acid, 1 m (100 mL)

£ sodium hydroxide, 1 m (100 mL)

£ sodium carbonate, 1 m (100 mL)

£ distilled or deionized water (boil and cool before use)

pH is specified on a log 10 scale, which allows a very wide
range of activities (concentrations) to be specified using
a small range of numbers. A difference of one pH number
corresponds to a difference of ten times in acidity or basicity.
For example, a solution with a pH of 5 is ten times (10^1 ) more
acidic than a solution with a pH of 6, and a solution with a pH
of 9 is ten times (10^1 ) more basic than a solution with a pH
of 8. Similarly, a solution with a pH of 2 is 10,000 (10^4 ) times
more acidic than a solution with a pH of 6, and a solution with
a pH of 12 is 10,000 (10^4 ) times more basic than a solution
with a pH of 8. Although the range of pH values is usually
considered to be 0 through 14, an extremely acidic solution
(such as a concentrated solution of hydrochloric acid) can
have a pH lower than 0, and an extremely basic solution (such
as a concentrated solution of sodium hydroxide) can have a pH
greater than 14.

For relatively dilute solutions of strong acids and bases, you can
estimate pH using the formula:

pH = –log 10 [H 3 o+]

where [H 3 O+] is the concentration of the hydronium ion in mol/L.
For example, hydrochloric acid dissolves in water according to the
following equation:

HCl + H 2 o → H 3 o+ + Cl–

Because HCl is a strong acid, the reaction proceeds to
completion, which is to say that essentially all of the HCl reacts to

form hydronium ions and chloride ions. The approximate pH of a

0.01 M solution of hydrochloric acid is:

pH = –log 10 [0.01] = 2

In calculating that approximate pH, we assume that the

hydrochloric acid fully dissociates in solution into H 3 O+ ions

and Cl– ions. For strong acids like hydrochloric acid, that’s a

reasonable assumption. For weak acids, such as acetic acid,

that assumption is not valid, because weak acids dissociate only

partially in solution. The concentration of hydronium ions in a

solution of a weak acid is lower (perhaps much lower) than the

concentration of the acid itself.

When acetic acid dissolves in water, the dissociation reaction

looks like this:

CH 3 CooH + H 2 o ⇔ H 3 o+ + CH 3 Coo–

This reaction reaches an equilibrium, with reactants being

converted to products and vice versa at the same rate. Therefore,

in a 1.0 M solution of acetic acid (about the concentration of

household vinegar), the actual concentration of the hydronium

ion is something less than 1.0 M, because some of the acetic acid