equal moles of which of the following chemicals could be used to make a basic buffer solution?

Buffer solution.....

A buffer is a mixture of a weak acid and its conjugate base.

The conjugate base of NH4^+ is NH3, which is ammonia. The resulting buffer will be basic with a pH of 9.26. Choice (d).

Choices (b) and (c) will not produce buffers. Choice (a) could produce an acidic buffer if fewer moles of OH- were used than moles of HF. But the question said they two species were equimolar.

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NH3(aq) + HOH(l) <==> NH4^+ + OH- ...................... Kb = 1.8x10^-5
Ka = Kw / Kb = 1.00x10^-14 / 1.8x10^-5 = 5.56x10^-10

Henderson-Hasselbalch equation
pH = pKa + log([base] / [acid])
pH = -log(5.56x10^-10) + log(1)
pH = 9.26 + 0
pH = 9.26 ................... the pH is expressed to two significant digits, in line with the Kb of NH3

When taking the log of a number, the precision is indicated by the digits to the right of the decimal. Therefore, a pH of 9.26 will have two significant digits.

A basic buffer solution is a solution mixture of a weak base and its conjugate acid.
(An acidic buffer solution is a solution mixture of a weak acid and its conjugate base.)

a) false
HF(aq) + NaOH(aq) → NaF(aq) + H₂O(l)
The resulting solution is the solution of the salt NaF. It is NOT a buffer solution.

b) false
The resulting solution is a mixture of STRONG acid HCl and its conjugate base Cl⁻. It is NOT a buffer solution.

c) false
There is no acid-base pair in the resulting solution. It is NOT a buffer solution.

d) true
The resulting solution is a mixture of WEAK BASE NH₃ and its conjugate acid NH₄⁺. It is a basic buffer solution.
(Actually, only this option is a buffer solution.)

The answer: d) NH3 and NH4Cl