Question

What is the approximate concentration of A3− ions in a 1.0 M aqueous solution of H3A? Assume that H3A has the following ionization constants at 25°C: Ka1 = 1x10−4, Ka2 = 1x10−5, Ka3 = 1.0x10−6

Answer 1

The first ionization of the acid H3A can be written as

Given that the initial concentration of the acid is , we can construct the following ICE table

Initial, M	1.0	0	0
Change, M	-x	+x	+x
Equilibrium, M	1.0-x	x	x

The equation for the equilibrium constant is

Using the ICE table for the equilibrium concentrations

Hence, the equilibrium concentrations are

Now, the conjugate base of H3A will undergo the second dissociation as follows:

Hence, we can create the following ICE table with the starting concentration of . and

Initial, M		0
Change, M	-y	+y	+y
Equilibrium, M		y

The equation for the second equilibrium constant is

Hence, using the ICE table

Hence,

The equilibrium concentrations are

Now, for the third dissociation

The ICE table can be written as

Initial, M		0
Change, M	-z	+z	+z
Equilibrium, M		z

hence, the equation for the equilibrium constant is

Note: the above can be easily calculated by assuming

From the ICE table

Hence, the approximate concentration of is