## CHM 112 Section 1

Homework Assignment #16
##### Due March 8, 2006 by 8:05 a.m.

Practice (not to be turned in): Problems 15.61

Find the equilibrium concentrations of oxalic acid, oxalate ion, hydrogen oxalate ion, and the pH of a solution of oxalic acid: the initial molar concentration of the oxalic acid in the solution is the eighth and ninth digits of your Student ID Number divided by 1000.

For the first step:

 H2C2O4(aq) + H2O(l) H3O+(aq) + HC2O4–(aq) Initial Ci 0 0 Change – x + x + x Equilibrium Ci – x x x

There is no approximation allowed in this step, so the solution requires use of the quadratic equation to solve the equation:

x2 + Ka1x – Ka1Ci = 0.

The positive solution to this gives the initial concentrations used in the second step, let this be Csec

For the second step:

 HC2O4–(aq) + H2O(l) H3O+(aq) + C2O42–(aq) Initial Csec Csec 0 Change – x + x + x Equilibrium Csec – x Csec + x x

In this case, approximation is allowed so:

[hydrogen oxalate ion] = [HC2O4] = Csec

[H3O+] = Csec

[oxalate ion] = [C2O42–] = Ka2

pH = –log[H3O+]