I'm guessing it's going to be

$$\ce{H2SO4 + 2 NaH2PO4 -> 2 H3PO4 + Na2SO4}$$

Could anyone please let me know what the actual reaction is?

Thank you

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    $\begingroup$ This will do.$\,$ $\endgroup$ – Ivan Neretin Sep 25 '18 at 20:11

Actually you have two reactions, since sulfuric acid can give off two protons:

$\ce{H2SO4 + NaH2PO4 -> NaHSO4 + H3PO4}$

$\ce{NaHSO4 + NaH2PO4 -> Na2SO4 + H3PO4}$

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:

$\ce{NaHSO4 + NaH2PO4 <=> Na2SO4 + H3PO4}$

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

| improve this answer | |
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    $\begingroup$ Well, it depends on concentrations too, but pKa2 for sulfuric acid is 1.99 and pka1 for phosphoric acid is 2.15. So for equal molarities of sulfuric and sodium dihydrogen phosphate you'd end up with a significant amounts of $\ce{NaHSO4 \text{,} Na2SO4 \text{,} NaH2PO4}$, and $\ce{H3PO4}$. $\endgroup$ – MaxW Sep 25 '18 at 20:29
  • $\begingroup$ That's why it's reversible. $\endgroup$ – Oscar Lanzi Sep 25 '18 at 20:31
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    $\begingroup$ Sorry, stated my point poorly. I'm trying to suggest that there is an equilibrium between the four species, and that all four species will likely have significant concentrations. (I say likely since I am too lazy to actually do the math...) $\endgroup$ – MaxW Sep 25 '18 at 20:37
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    $\begingroup$ Ok, it's a two-step reaction Thank you, Oscar Lanzi and MaxW for the information $\endgroup$ – praMATH Sep 25 '18 at 21:03

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