Chemistry Reference
In-Depth Information
water
HCl
(s)
H
+
(aq)
+ Cl
-
(aq)
The bare protons (H
+
) that the acid releases are too reactive to exist as
isolated particles for any real length of time. They will quickly attach them-
selves either to hydroxide ions to form neutral water molecules, or to neu-
tral water molecules to form additional hydronium ions, according to the
reaction shown here:
H
+
+ H
2
O
H
3
O
+
hydrogen ion + water
hydronium ion
The result of adding the acid to the water will be an increase in hydro-
nium ions and a corresponding decrease in hydroxide ion. The product of
the concentrations of hydronium ions and hydroxide ions is a constant, called
the ion-product constant for water, which is symbolized as k
w
. The formula
for this constant is shown here:
k
w
= [H
3
O
+
] × [OH
-
] = 1.0 × 10
-14
M
This equation shows us that the product of the concentrations of these
ions must always be the same, 1.0 × 10
-14
. This gives us the ability to calcu-
late the concentration of one type of ion, when we know the concentration
of the other type of ion. For example, if we know that the hydronium ion
concentration of the HCl acid solution that we “created” was 1.0 × 10
-2
M,
we could solve for the concentration of OH
-
ions, as shown here:
k
w
[H
3
O
+
]
1.0 × 10
-14
1.0 × 10
-2
= 1.0 × 10
-12
[OH
-
] =
=
To check your answer, make sure that the product of the concentra-
tion of hydronium ions and hydoxide ions is equal to 1.0 × 10
-14
, as shown
here:
k
w
= [H
3
O
+
] × [OH
-
] = (1.0 × 10
-2
) × (1.0 × 10
-12
) = 1.0 × 10
-14
A basic solution is one that has a greater concentration of OH
-
ions
than H
3
O
+
ions. How could this come to pass? Let's suppose that we started
out with our pure water, with a pH of 7. Remember: This pure water has
an equal number of hydronium ions and hydroxide ions. Then, we take
some pure sodium hydroxide (NaOH) and dissolve it in our pure water.
The sodium hydroxide will ionize according to this equation:
water
NaOH
(s)
Na
+
(aq)
+ OH
-
(aq)
