General Chemistry Principles, Patterns, and Applications, 2011

Given: volume and concentration of acid and base
Asked for: pH
Strategy:
A Calculate the initial millimoles of the acid and the base. Use a tabular format to determine the amounts
of all the species in solution.
B Calculate the concentrations of all the species in the final solution. Use Equation 16.16 to determine [H + ]
and convert this value to pH.
Solution:
A Table 16.4 "Values of p" gives the pK a values of oxalic acid as 1.25 and 3.81. Again we proceed by
determining the millimoles of acid and base initially present:
100.0 mL(0.0510 mmol H2oxmL) = 5.10 mmol H2ox
55.0 mL(0.120 mmol NaOHmL) = 6.60 mmol NaOH
The strongest acid (H 2ox) reacts with the base first. This leaves (6.60 − 5.10) = 1.50 mmol of OH − to react
with Hox − , forming ox 2− and H 2O. The reactions can be written as follows:
H2ox5.10 mmol +OH - 6.60 mmol ® Hox - 5.10 mmol + H2O5.10 mmol
Hox - 5.10 mmol +OH -1.50 mmol ® ox2 -1.50 mmol + H2O1.50 mmol
In tabular form,
H2ox OH − Hox − ox 2−
initial 5.10 mmol 6.60 mmol 0 mmol 0 mmol
change (step 1) −5.10 mmol −5.10 mmol +5.10 mmol
0 mmol
final (step 1) 0 mmol 1.50 mmol 5.10 mmol 0 mmol
change (step 2)
— −1.50 mmol −1.50 mmol +1.50 mmol
final 0 mmol 0 mmol 3.60 mmol 1.50 mmol
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