Chemistry Reference
In-Depth Information
K
a
of HF = 3.5 × 10
-4
= 0.00035
K
a
of H
3
PO
4
= 7.5 × 10
-3
= 0.0075
K
a
of H
2
O = 1.0 × 10
-14
= 0.00000000000001
The largest of these numbers is 0.0075, making the H
3
PO
4
our stron-
gest acid. The number 0.00035 is the next largest, so the HF would be the
next strongest acid. The water, with a K
a
value of 0.00000000000001, is our
weakest acid, which shouldn't come as much of a surprise. We can use this
information to answer the first part of Example 1.
A. List the three acids in order from strongest to weakest.
Answer: A. H
3
PO
4
, HF, H
2
O
In order to answer part B of the question, we need to find the conju-
gate bases of all three acids. You will remember from Lesson 9-4 that the
conjugate base of an acid is the substance that is left after the acid donates
a proton. When an acid loses a proton, we take one “H” away from its
formula, and we decrease its charge by +1. So,
H
3
PO
4
becomes H
2
PO
4
-
HF becomes F
-
H
2
O becomes OH
-
In order to arrange them from strongest base to weakest base, we need
only remember that the stronger an acid is, the weaker its conjugate base
will be. Therefore, the order of our bases will be the opposite of the order
of our acids.
A. List the three acids in order from strongest to weakest.
B. List the conjugate bases of the three acids in order of
strongest to weakest.
Answer:
A. H
3
PO
4
, HF, H
2
O
B. OH
-
,F
-
, H
2
PO
4
-
When you are asked to solve problems involving acids and bases, you
need to pay attention to their strengths, even if the word strength isn't men-
tioned in the problem. Strong acids and bases are treated as if they ionize
completely, so the concentration of the ions in an aqueous solution of a
strong acid or base is directly related to the molarity of the acid or base. If
a question asks for the concentration of H
3
O
+
ions in a 0.001 molar solu-
tion of HCl, the answer would be 0.001 M, because each molecule of HCl,
a strong acid, is thought to provide one hydronium ion.
