<p>I've been studying acid-base equilibria and I don't get this question:
Acid Ka
H3PO4 7 x 10¯3
H2PO4¯ 8 x 10¯8
HPO42¯ 5 x 10¯13</p>
<p>On the basis of the information above, a buffer with a pH = 9 can best be made by using</p>
<pre><code>(A) pure NaH2PO4
(B) H3PO4 + H2PO4¯
(C) H2PO4¯ + PO43¯
(D) H2PO4¯ + HPO42¯
(E) HPO42¯ + PO43¯
</code></pre>
<p>The answer is D, but why?</p>
<p>The best buffers have a pKa = pH.</p>
<p>I calculated the PKa's though and didn't get 9 for any of them.</p>
<p>I'm not sure how to do this, but the Zumdahl Chemistry book has this on Chapter 15 (of the 7th edition, I believe).</p>
<p>My class is on the chapter right now.</p>
<p>because one is slightly over and the other is slightly under therefore ud require the least out of those two to make a ph of 9...</p>