<li>Equal volumes of 0.10molar H3PO4 and 0.20molar KOH are mixed. After equilibrium is established, the type of ion an solution in largest concentration, other than the K+ ion, is
a. H2PO4<br>
b. HPO42<br>
d. OH
c. PO43<br>
e. H3O+</li>
</ol>
<p>Answer is B and only 15% got it right on the exam. Can someone explain why it’s B?</p>
<p>not 100% on this but pretty sure</p>
<p>first start with the initial reaction which creates .10 moles of H2PO4 and .10 moles of OH ions and has .10 moles of KOH left over. the reaction is not complete and the H2PO4 can react with the left over KOH to create .10 moles of HPO4</p>
<p>basically since KOH is twice the molar concentration of H3PO4 it can react twice to take off 2 hydrogens to reach equilibrium</p>
<p>what mjgil said is correct. </p>
<p>H3PO4 is a multiprotic acid(meaning it has more than one proton to lose), and it can be inferred from the problem that H2PO4- is also a strong acid.</p>
<p>Don't mean to be picky, but neither H3PO4 nor H2PO4- are strong acids. mjgil's explanation is mostly correct, but when .10 moles KOH initially reacts with H3PO4 it creates .10 moles of H2PO4 and .10 moles of H2O (not .10 moles of OH ions). Then the .10 moles of OH that's left reacts with the H2PO4(1-) and converts it to HPO4(2-)</p>