You are an environmental chemist studying a local waterway and need to know the amounts of all forms of phosphate in the stream. You might begin by making up some solutions of phosphoric acid to use as standards. Calculate the concentrations of all solute species in 0.10 m H₃PO₄(aq).

ANTICIPATE Because successive deprotonations result in ever weaker acids, you should expect the concentrations to be in the order H₃PO₄ > H₂PO₄^– > HPO₄ ^2– > PO₄ ^3–, with very, very little of the completely deprotonated species. Because the solution is acidic, you should also expect the concentration of OH^– to be very small.

PLAN Follow the procedure for a triprotic acid in Toolbox 6E.1.

What should you assume? Assume that only the first deprotonation affects the pH and that the autoprotolysis of water has no significant effect on the pH.

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Toolbox 6E.1 HOW TO CALCULATE THE CONCENTRATIONS OF ALL SPECIES IN A POLYPROTIC ACID SOLUTION CONCEPTUAL BASIS Assume that the polyprotic acid, a weak acid for all its deprot- onations, is the solute species present in largest amount. Assume that only the first deprotonation contributes significantly to [H3O+] and that the autoprotolysis of water does not contribute significantly to [H3O+] or [OH-]. PROCEDURE FOR A DIPROTIC ACID Step 1 From the deprotonation equilibrium of the acid (H₂A), determine the concentrations of the conjugate base (HA) and H3O+, as illustrated in Example 6D.1. Step 2 Find the concentration of A² from the second deprotonation equilibrium (that of HA) by substituting the concentrations of H3O+ and HA from step 1 into the expression for K₂2. Step 3 Find the concentration of OH by dividing K, by the concentration of H₂O*. PROCEDURE FOR A TRIPROTIC ACID Step 1 From the deprotonation equilibrium of the acid (H₂A), determine the concentrations of the conjugate base (H₂A) and H₂O¹.