First of all, it helps to write the chemical formulas for all the reactants.
NH4Cl = ammonium chloride
NaH2PO4 = sodium dihydrogen phosphate
Next, describe the way these salts dissolve, forming ions.
NH4Cl -> NH4(+) + Cl(-)
NaH2PO4 -> Na(+) + H2PO4(-2)
These are the main ions found in a solution of these two salts. No reactions will occur because all the ions are soluble and none of the potential salts formed by combining ions are insoluble, either. However, the ammonium ion and phosphate ions are weak acids and bases. This means molecules may form, leaving just a few ions in solution. For the ammonium and phosphate ions, an equilibrium is established with water:
NH4(+) + H2O <-> NH4OH + H(+)
H2PO4(-) + H2O <-> H3PO4 + OH(-)
The actual amount of ammonia or phosphoric acid depends on something called an equilibrium constant. This is a ratio of molecules to ions and is constant, meaning if some of the ions are removed from solution, some of the molecules form more ions to replace them. For the simple weak acid HA:
HA -> H(+) + A(-)
the equilibrium constant is given by:
Ksp = [H(+)][A(-)]/[HA]
For typical weak acids and bases, equilibrium constants are tiny, meaning only very small amounts of ions form. Therefore for the weak acid HA, the equilibrium is shifted far to the left, once again meaning most of the substance exists as an inert molecule.
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