Question

Use the method of successive approximations to determine the pH and concentrations of H2A, HA–, and A2– in a solution of 0.00296 M monopotassium fumarate (KHA). The pKa values for fumaric acid are 3.02 (pKa1) and 4.48 (pKa2).

Answer 1

Diprotic acid: A weak diprotic acid, , is a chemical substance which donates two hydrogen () ions successively when dissolved in water. The common examples of weak diprotic acids are oxalic acid, sulfuric acid, and carbonic acid.

Power of hydrogen ions (pH):The pH is defined as the negative logarithmic function of concentration of hydrogen () ion in an aqueous solution.

Molar concentration: Molar concentration is the number of moles of solute that will dissolve in a liter of solution. It is expressed as mol/L or M (molarity).

Fundamentals

Molar concentration:

Two stages of dissociations:

The equation for the pH of the solution:

Henderson-Hasselbalch Equation:

Some important relations of:

An ICE table is used to find the concentrations or moles of individual reactants or products at equilibrium.

The expansion of an ICE table is as follows:

All quantities are expressed in terms of concentrations or moles.

The information in the problem is extracted as follows:

[ HA-] = 0.00296 M

Assume:

Using these conditions

A^2- = 10^-4.48 M = 3.31*10^-5 M

pH = 0.5(pKa1 - lg(H2A)) = 3.02
so
H2A = 9.5510^-4 M