A solution is made by adding potassium phosphate (K_3PO_4 (MW = 134 g/mol; EW =

Question

A solution is made by adding potassium phosphate (K_3PO_4 (MW = 134 g/mol; EW = 44.67 g/eq)) to pure water to give a concentration of 10^-1 M. Assume the K_3PO_4 completely dissolves and dissociates upon addition to water (it is a salt). (a) Write all chemical equilibrium reactions for this solution. (b) Using the law of mass action, write all chemical equilibrium expressions (equations) for this solution. (c) Write the mass balance equation(s) for this solution. (d) Write the charge balance for this solution. (e) Estimate the pH of the solution.

Explanation / Answer

a) Given that K3PO4 completely dissolves and dissociates upon addition to water.

Hence,

K3PO4 + 3H2O <------------> H3PO4 + 3 KOH

b) K3PO4 + 3H2O <------------> H3PO4 + 3 KOH

     Equilibrium expression is

     Equilibrium constant K = concentration of products at equilibrium/ concentration of reactants at equilibrium

                                       = [H3PO4] [ KOH]3/ [K3PO4 ] [H2O]3

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