Question

.
Using the volume of consumed sodium hydroxide, determine the
concentration of unknown phosphoric acid at each of the end points
Ka1 and Ka 2.

Answer 1

Answer:

three equilibrium reactions define the system.

H3cit+H2O↔H2cit−+H3O+H2cit2−+H2O↔Hcit2−+H3O+Hcit−+H2O↔cit3−+H3O+Ka1=7.45×10−4Ka2=1.73×10−5Ka3=4.02×10−7(1)(2)(3)

Even though the first two Ka values are fairly close to each other, we can still use only the Ka1 expression to solve for the initial pH.

InitialEquilibriumApproximationH3cit+H2O↔0.10.1−x0.1H2cit−+0xxH3O+0xxKa1(4)(5)(6)(7)

Ka1=[H2cit−][H3O+][H3cit]=(x)(x)0.1=7.45×10−4(8)

x=[H3O+]=0.00863pH=2.06(9)

If we check the approximation, it actually turns out that the value is too high and that