Chemistry Reference
In-Depth Information
Before we go any further, I would like to remind you that when you see
a species in brackets, as in [H
+
], it is referring to the concentration of the
species, usually in moles/L. Therefore, if we say that [H
+
] = 1.0 × 10
-7
, we
mean the concentration of hydrogen ions is .0000001 M. I would also like
to point out that, in this section of the topic, you should consider H
3
O
+
to
be synonymous with H
+
. Many topics describe acids in terms of hydrogen
ions, [H
+
], but others argue that hydrogen ions, which are essentially bare
protons, won't exist for any significant length of time in water. They will
attach to water molecules, forming hydronium ions, H
3
O
+
. You must not
get confused when switching between chemistry topics. Whether we dis-
cuss acids in terms of hydrogen ions (H
+
) or hydroxide ions (H
3
O
+
), we are
really talking about the same thing.
The pH and pOH scales are shorthand notations, in a sense. Instead of
writing that the concentration of hydronium ions in a particular solution is
0.00001 M (or, 1 × 10
-5
M, in scientific notation), we simply say that the pH
of the solution is 5. How do we convert between the concentration of the
ions and the pH or pOH scales? We simply use the following formulas.
pH = -log[H
3
O
+
]
pOH = -log[OH-]
To start with, try to use the pH formula to find the pH of a solution
with a hydronium ion concentration of 1× 10
-5
M. I already told you that it
comes out to a pH value of 5. Can you get that answer on your calculator? Of
course, each calculator is different, so I can't instruct you on how to use your
specific calculator, but I can give you some tips. First, check to see if your
calculator has a “log” button. If it doesn't, you will need a different calcula-
tor to solve these problems. Second, to get the negative sign in front of the
“log” in the formula, don't use the subtraction button. Look for another but-
ton that may have a smaller minus sign, perhaps in parentheses, such as (-).
We solve the problem as follows;
pH = -log[H
3
O
+
] = -log(1 × 10
-5
) = 5
A little trick that you should learn early on (especially if your calcula-
tor lacks a “log” button) is that when the coefficient in your concentration
of hydronium ions is 1, as in 1 × 10
-8
, the pH is equal to the absolute value
of the exponent. The pOH scale is used in a similar way to indicate the
concentration of hydroxide ions in a solution. A concentration of hydrox-
ide ions of 1 × 10
-3
would correspond to a 3 on the pOH scale. The follow-
ing table summarizes the method of converting the concentrations of
hydronium ions and hydroxide ions to the pH and pOH scales respectively.
