## Chemistry: The Molecular Science (5th Edition)

$NH_3$ and $NH_4Cl$ are the best combination for a buffer with $pH \approx 9$ because, the $pKa$ for $N{H_4}^+$ is about $9$.
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.