Free energy is the amount of energy that can do work.
Free energy is the internal energy of a system less the amount of energy that cannot perform work.
<h3>
Answer</h3>
HF
<h3>Explanation</h3>
A buffer solution contains <em>a weak acid</em> and<em> its conjugate base</em>. The two species shall have a similar concentration in the solution. It's also possible for <em>a weak base</em> and <em>its conjugate acid</em> to form a buffer solution.
The KF solution already contains large number of
ions. The objective is to thus find its conjugate acid or base.
contains no proton
and is unlikely to be a conjugate acid. Assuming that
is a conjugate base. Adding one proton to
would produce its conjugate acid.

Therefore
is the conjugate acid of
.
happens to be a weak acid. As a result, combining
with
would produce a solution with large number of both the weak acid and its conjugate base, which is a buffer solution by definition.
"only one kind of atom" as in only one element, right?
That would be a diatomic molecule.
Molar mass:
( Br₂ ) = 159.80 g/mol
AlCl₃ = 133.34 g/mol
<span>2 AlCl</span>₃<span> + 3 Br</span>₂<span> = 2 AlBr</span>₃<span> + 3 Cl</span>₂
2 x ( 133.34) g ------------ 3 x ( 159.80) g
11.95 ------------------------ ( mass Bromine )
mass Bromine = 11.95 x 3 x 159.80 / 2 x 133.34
mass Bromine = 5728.83 / 266.68
mass Bromine = 21.48 g
hope this helps!