Answer
$NH_3$ and $NH_4Cl$ are the best combination for a buffer with $pH \approx 9$ because, the $pKa$ for $N{H_4}^+$ is about $9$.
Work Step by Step
1. Identify the acid:
$N{H_4}^+$.
2. Calculate its $pK_a$
** Use the Kb from its conjugate base.
$K_b * K_a = K_w = 10^{-14}$
$ 1.8\times 10^{- 5} * K_a = 10^{-14}$
$K_a = \frac{10^{-14}}{ 1.8\times 10^{- 5}}$
$K_a = 5.556\times 10^{- 10}$
$pKa = -log(Ka)$
$pKa = -log( 5.556 \times 10^{- 10})$
$pKa = 9.25$
3. Compare this value to 9.
- 9.25 is close to 9, therefore, this combination is a good choice.
