What's the reaction between H2SO4 and NaH2PO4?
Actually you have two reactions, since sulfuric acid can give off two protons:

$\ce{H2SO4 + NaH2PO4 -&gt; NaHSO4 + H3PO4}$

$\ce{NaHSO4 + NaH2PO4 -&gt; Na2SO4 + H3PO4}$

The $\ce{NaHSO4}$ is called sodium bisulfate.  Bisulfate is the thing you form first when sulfuric acid acts as an acid, then you get the sulfate ion if you have a strong enough base to react with the almost but not quite strongly acidic bisulfate ion.  In this case, actually, you can look up some acid dissociation constants and discover that phosphoric acid has almost exactly the same amount of strength as the bisulfate ion acting as the "second stage" of sulfuric acid.  So the second reaction is really reversible:

$\ce{NaHSO4 + NaH2PO4 &lt;=&gt; Na2SO4 + H3PO4}$

For more on what makes acid-base reactions reversible, see here.