The pH at the first equivalence point in the titration of oxalic acid with sodium hydroxide is determined by the concentration of remaining protons from the first deprotonated form of the acid. Considering the given pKa values, the pH should be between 1.23 and 3.81, making 3.45 the most reasonable answer for the question provided.
The pH at the first equivalence point in the titration of oxalic acid (H2C2O4) with sodium hydroxide (NaOH) occurs when all the first acidic protons have reacted with the hydroxide ions to form the conjugate base (HC2O4-). The pKa1 of oxalic acid is given as 5.9x10-2, which can be converted to a pKa value by taking the negative logarithm of the Ka value, resulting in a pKa1 value of approximately 1.23. At the first equivalence point, the solution will essentially contain only the conjugate base form of oxalic acid, and its pH will be determined by the concentration of the remaining protons in the solution.
Using the ICE (Initial, Change, Equilibrium) approach and the given second ionization constant (Ka2 = 6.4x10-5), we can calculate the pH when half of the second acidic protons have been titrated, which corresponds to the first equivalence point. This will give a pH that is greater than the first pKa1 but less than the second pKa2 of oxalic acid. Given that pKa2 corresponds to a pH of 3.81, and the fact that we are concerned with the first equivalence point (which is just after all of the first protons are titrated and before any of the second protons are significantly titrated), the pH value will be between 1.23 and 3.81.
The closest value from the options provided that falls between 1.23 and 3.81 is 3.45, which suggests that this pH value is the most reasonable answer for the first equivalence point in the titration of oxalic acid with sodium hydroxide (NaOH).
The complete question is- What is the pH at the first equivalence point in the titration of oxalic acid, H2C2O4 with sodium hydroxide, NaOH? The Ka1 = 5.9x10-2 and the Ka2 = 6.4x10-5.
a.) 2.71
b.) 3.45
c.) 6.14
d.) 8.15