Chapter 11 Laboratory: Acid-Base Chemistry 195Solving for x gives us a value of about 4.17 · 10–3 or about 0.00417.
Our original estimate that 10% of the acetic acid would dissociate
was wildly high. In fact, only about 0.417% of the acetic acid
dissociates. Knowing that value tells us that the concentration
of the hydronium ion in a 1 M acetic acid solution is 0.00417 M.
Plugging that value into the formula allows us to calculate the
approximate pH of the 1.0 M acetic acid solution:
pH = –log 10 [0.00417] = 2.38
Although we’ve focused until now on acids rather than bases,
remember that the concentrations of the hydronium ion and
the hydroxide ion are related. We know that pure water at 25°C
has a pH of 7.00 and that the concentrations of hydronium
ions and hydroxide ions are equal. A pH of 7.00 tells us that the
concentration of hydronium ions, [H 3 O+], is 1.00 · 10–7, which
means that the concentration of hydroxide ions, [OH–], must also
be 1.00 · 10–7. We know that hydronium ions and hydroxide ions
react to form water:
H 3 o+ + oH– ⇔ H 2 o
According to Le Chatelier’s Principle (see Chapter 13), in a
system at equilibrium, increasing the concentration of one
reactant forces the reaction to the right, producing more product.
Increasing [H 3 O+] reduces [OH–] proportionately, and vice versa.
Expressed as a formula, the equilibrium constant is:
k = [H 3 o+] · [oH–]
or
k = (1.00 · 10–7) · (1.00 · 10–7) = 1.00 · 10–14
In other words, the product of the concentrations of the
hydronium ions and the hydroxide ions always equals 1.00 · 10–14.
CUTIOA nS
All of the chemicals used in this laboratory can be
hazardous, particularly in concentrated solutions.
Check the MSDS for each of these chemicals before you
proceed. Wear splash goggles, gloves, and protective
clothing at all times.z
kA vRSUSpE kA
The acidity constant may be specified directly as Ka or as
the negative logarithm of Ka, which is abbreviated as pKa.
For example, at 25°C, the Ka of acetic acid is 1.74 · 10–5 and
the pKa is 4.76. Either value can be used by adjusting your
calculations accordingly.dAUL. Rp joNES CommENTS
I’d say that pKa is used far more today than Ka. Also, you
might mention that any compound with hydrogen can,
plausibly, be an acid. However, if the pKa of the compound
is above 14, it essentially won’t dissociate at all in water.
Also, when pH = pKa, the acid is 50% dissociated; another
good reason to use pKa.If you increase the concentration of hydronium ions by a factor of
10 (or 10,000), the concentration of hydroxide ions decreases by
a factor of 10 (or 10,000), and vice versa.In this laboratory, we’ll determine the pH of solutions of two
strong acids, a weak acid, a strong base, and a weak base at
various concentrations.POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.- Label six beakers #1 through #6.
- Pour about 100 mL of 1 M hydrochloric acid into
beaker #1. - Using the 10 mL pipette, transfer 10.00 mL of the 1 M
hydrochloric acid to the 100 mL volumetric flask. - Fill the volumetric flask to the reference line with distilled
water, mix thoroughly, and transfer the 0.1 M solution of
hydrochloric acid to beaker #2. - Proceeding from beaker to beaker, repeat steps 4 and 5
until you have 0.01 M, 0.001 M, 0.0001 M, and 0.00001
M solutions of hydrochloric acid in beakers #3 through
#6, respectively. - Read and follow the directions for your pH meter with
respect to calibrating it, rinsing the electrode between
measurements and so on. Use the pH meter to measure
the pH of the solutions in beakers #1 through #6, and
record those values on line A of Table 11-1. - Repeat steps 3 through 7 for the solutions of sulfuric
acid, acetic acid, sodium hydroxide, and sodium
carbonate. - Based on your observations, calculate the pKa values
for the acids and the pKb values for the bases, and enter
your calculated values in Table 11-1.
dISpoSAL: Retain the 1.0 m acid and sodium hydroxide
solutions for use in the following lab. Neutralize the
other solutions, beginning with the most dilute samples,
by pouring them into a bucket or similar container. The
contents of the bucket can be flushed down the drain.