# Chapter 17 - Questions and Problems - Page 825: 17.20

The student should choose the $HC$, because it has the closest $pK_a$ to $8.60$.

#### Work Step by Step

1. Calculate the pKa value for $HA$. $pKa = -log(Ka)$ $pKa = -log( 2.7 \times 10^{- 3})$ $pKa (HA) = 2.569$ 2. Calculate the pKa value for $HB$ $pKa = -log(Ka)$ $pKa = -log( 4.4 \times 10^{- 6})$ $pKa (HB)= 5.357$ 3. Calculate the pKa value for $HC$: $pKa = -log(Ka)$ $pKa = -log( 2.6 \times 10^{- 9})$ $pKa(HC) = 8.585$ Therefore, $HC$ is the better choice for $pH = 8.60$, because it has the closest $pK_a$.

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