Review Questions
pH
1. Explain dissociation.
A tiny fraction of water molecules in a liquid will spontaneously break
apart into ions. Ions are charged atoms or molecules. During
dissociation a water molecule breaks apart into a free H+ (hydrogen ion
or proton) and a free OH- (hydroxide ion).
H2O → H+ + OHChemists have measured how much of a liter of water is made up of
these dissociated water molecules. In a liter of pure water, there is:
1 x 10-7 moles of H+ and 1 x 10-7 moles of OHWater isn’t the only compound that will dissociate. Acids and bases do
as well. Acids, like HCl, will dissociate into separate ions (e.g. H+ and
Cl-) when mixed with water. Since an acid releases more H+ than OH-,
acids are proton donors. We define an acid as a compound that when
mixed with water releases an excess amount of H+.
HCl → H+ + ClBases on the other hand are defined as compounds that reduce the H+
concentration of a solution. Sodium hydroxide (NaOH), also known as
lye, is a common base. Like an acid, it will also split apart in water
releasing Na+ and OH- when mixed with water. Since water has a small
quantity of free H+, the extra OH- from the lye will bind to the H+ and
reform water. In this case, the lye has reduced the concentration of H+
in the solution. Another example is ammonia (NH3). Ammonia doesn’t
dissociate but in water it will also bind to free H+ and form an
ammonium ion (NH4+) and so reduce the H+ concentration.
NaOH → Na+ + OHH2O → H+ + OHH+ + OH- → H2O
2. What is the difference between a strong acid and a weak acid or a
strong base and weak base?
The major difference is the amount of dissociation that occurs. Both
strong acids and strong bases will dissociate almost completely. They
significantly increase or reduce the H+ concentration in a solution. Both
HCl and NaOH are strong. Weak acids or weak bases don’t dissociate
as much. So there impact on H+ concentration is less.
3. What is pH?
Since chemists can measure the concentration of H+ in a solution, you
can compare the concentration levels of many acids and bases. If you
recall from a previous question, the H+ concentration is measured in
moles per liter. Remember, the concentrations are often tiny. So
working with them requires using exponents. We know for example
that water has a H+ concentration of 1 x 10-7 and vinegar (a weak acid)
has a H+ concentration of 1 x 10-3. Now that is a reasonable
comparison but no one really likes to play with exponents, so a couple
of hundred years ago a Danish chemist named Sorenson converted
these exponents into whole numbers and created the pH scale.
How the conversion works is this. Let’s take water for instance. It has a
concentration of 1 x 10-7 moles per liter. Well, what Sorenson did was
take this number and transformed it into a negative natural logarithm.
Instead of 1 x 10-7, you now just have 7. So water has a pH of 7. If we
go back to vinegar with its H+ concentration of 1 x 10-3, the pH would
be 3. (See figure and table on next page)
Since water is our standard, we can make it the middle number of our
pH scale. Water is pH 7 and is called neutral. Acids are solutions that
have a pH less than 7 and bases have a pH greater than 7. Solutions
that are basic are also known as alkaline.
What is the difference in H+ concentration between, let’s say, a pH of 7
and a pH of 6. Well since this is a logarithmic scale, it is 10x difference
between pH numbers. For instance, there are 10x more H+ in pH 6
than pH 7. Between a pH of 5 and a pH of 7, there would be 100x
more H+. It is the same going the other way. A pH of 8 would have 10x
less H+ than pH 7. A pH of 9, 100x less than pH 7. If we look at the
typical pH scale. Acids range from 0-7 and bases from 7-14. An acid
with a pH of 1 has one million times more H+ than water and a base at
pH 14 has one million times less.
Acidic
(H donors)
+
Neutral
Basic (Alkaline)
(H+ acceptors)
H+ Concentration
(Moles/Liter)
1 x 100
1 x 10-1
1 x 10-2
1 x 10-3
1 x 10-4
1 x 10-5
1 x 10-6
1 x 10-7
1 x 10-8
1 x 10-9
1 x 10-10
1 x 10-11
1 x 10-12
1 x 10-13
1 x 10-14
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
OH- Concentration
(Moles/Liter)
1 x 10-14
1 x 10-13
1 x 10-12
1 x 10-11
1 x 10-10
1 x 10-9
1 x 10-8
1 x 10-7
1 x 10-6
1 x 10-5
1 x 10-4
1 x 10-3
1 x 10-2
1 x 10-1
1 x 100
4. Why does unpolluted rainwater have an acidic pH of 5.6?
We have made water our standard for the pH scale. Water has a pH of
7. Then why would unpolluted rainwater have a lower pH (5.6)? Water
in the atmosphere mixes with CO2 and produces a weak acid called
carbonic acid (H2CO3). We can replicate this process by examining
seltzer water. Seltzer is simply carbonated water (dissolved carbon
dioxide in the water). It also has acidic pH. Just like in the atmosphere,
some of the water with combine with CO2 and make carbonic acid.
Seltzer has a slight tangy flavor because acids taste sour.
H2O + CO2 = H2CO3
5. What is acid rain?
Rainwater that has a pH less than 5.6 is called acid rain. Pollutants
from the burning of fossil fuels, like sulfur dioxide (SO2), mix with water
in the atmosphere and create a strong acid (like sulfuric acid H2SO4).
Acid rain can be damaging to plants and wildlife. Whole forests have
been killed. Lakes and streams have been acidified making them
uninhabitable for fish and invertebrates.
H2O + SO2 = H2SO4
6. What is neutralization?
You can return the pH of a solution to neutral by adding the opposite
pH of the solution. For example, if you have a strong basic solution,
you can neutralize the base by adding an acid. Same goes for acid
solutions, you can neutralize them by adding a base. In fact, antacids
that people take for heartburn are bases.
NaOH → Na+ + OHHCl → H+ + ClH+ + OH- → H2O
Na+ + Cl- → NaCl
7. How can pH be held constant in changing conditions?
Living organisms have to keep their body fluids within a narrow pH
range. Our blood, for example, must remain within a pH range of
7.35-7.45. You can prevent pH fluctuations by using a buffer. A buffer
is a compound that will donate H+ when a solution becomes too
alkaline and reduce H+ concentration when it is too acid. In our body
we have three major buffering systems. The most important is the
bicarbonate system. It uses a weak acid for donating H+ (carbonic acid
—H2CO3) and a weak base for absorbing H+ (sodium bicarbonate-NaHCO3). This buffering system works by converting strong acids and
bases into weak ones, thereby, reducing the potential fluctuation of the
pH.