Which of the following is an
acid–base conjugate pair? (A) Na2CO3 and CO32−, (B) C2H4 and
C2H6, (C) NO3− and HNO3, (D) K+ and Br−, (E) H2 and H+.
When you have an acid or base
that’s in water, the acid or base can react with the water. In this case, we have hydrofluoric
acid, which would react with water to form H3O+, or the hydronium ion, and F−, or
the fluoride ion. Sometimes you’ll see this type of
reaction simplified like this, where we just have the hydrofluoric acid dissociating
to form the H+ ion and the F− ion. But this is just a simplification
as the H+ ion would immediately react with water to form hydronium. And there wouldn’t be any H+ ions
remaining in the solution after a very short period of time.
If we look in detail at this
reaction, we’ll notice that hydrofluoric acid loses a proton, or the H+ ion, over
the course of the reaction. This proton that the hydrofluoric
acid loses is gained by the water when it forms hydronium. Since hydrofluoric acid loses a
proton over the course of the reaction, it’s a Brønsted–Lowry acid, which is a
chemical species that donates a proton.
Similarly, water in this reaction
would be a Brønsted–Lowry base since it’s a chemical species that accepts a proton
over the course of the reaction. So when a Brønsted–Lowry acid or a
Brønsted–Lowry base reacts, we’ll end up with a pair of chemical species that differ
from each other by a proton, which we call an acid–base conjugate pair.
So in our reaction, our
hydrofluoric acid and the fluoride ion are an acid and a base that differ from each
other by a proton. The same is true for water and the
hydronium ion. Again, they are an acid and a base
that differ from each other by a proton. So they’re also an acid–base
So we can identify an acid–base
conjugate pair by finding an acid and a base that differ from each other by a proton
or the H+ ion. We can see the same thing in the
reaction of the weak base ammonia with water, which would form hydroxide and the
weak acid ammonium. Ammonia and ammonium differ from
each other by a proton. So they’re an acid–base conjugate
pair. Same would be true for water and
the hydroxide ion.
So now that we understand what an
acid–base conjugate pair is, let’s look through our answer choices. Our first answer choice is Na2CO3,
or sodium carbonate, and CO32−, which is the polyatomic anion carbonate. Though both of these have the
carbonate ion in common, and carbonate can act as a base, these two chemical species
clearly do not differ by a proton. So they’re not an acid–base
Our next answer choice is C2H4,
which is called ethene, and C2H6, which is called ethane. Both are organic molecules. Though these two molecules are
similar and differ from each other by some hydrogens, neither of them will act as an
acid or a base in water. So this is not the answer choice
that we’re looking for either.
Our next answer choice is NO3–,
which is the polyatomic anion nitrate, and HNO3, which is called nitric acid. Nitric acid is a strong acid that
can react with water to form the hydronium ion and nitrate. Nitric acid is definitely an
acid. And HNO3 and NO3– differ from each
other by a proton. This makes them an acid–base
conjugate pair, which means that answer choice (C) is the answer that we’re looking
It’s worth mentioning that since
nitric acid is a strong acid, its conjugate base, NO3–, won’t be a very good base
since the conjugate bases of strong acids are very weak bases. This means that the nitrate ion
will react much less than the conjugate bases of weaker acids, like the conjugate
base of the hydrofluoric acid that we were looking at earlier, the fluoride ion.
So we’ve identified our correct
answer. But let’s quickly look through our
two remaining answer choices just so we understand why they’re not an acid–base
conjugate pair. Answer choice (D) features the
potassium cation and the bromide anion. These two chemical species clearly
do not differ by a proton, which is why answer choice (D) is incorrect.
Our final answer choice is H2, or
hydrogen gas, and H+, or the hydrogen cation. Though this answer choice may look
tempting, since the difference between H2 and H+ is a hydrogen, it would in fact be
the hydrogen anion, not a proton. Additionally, H2 gas is not an acid
or a base. When dissolved in water, no
reaction occurs. So H2 and H+ are not an acid–base
Of our answer choices, the only one
that was an acid–base conjugate pair was NO3– and HNO3, and answer choice (C).