24) It was found that a 20.00 mL portion of a solution of oxalic acid, H2C2O4, requires 6.69 mL of 0.200 M K2Cr2O7 for complete reaction in an acidic solution. In the reaction, the oxidation product is CO2 and the reduction product is Cr3+. How many milliliters of 0.450 M NaOH are required to completely neutralize the H2C2O4 in a separate 20.00 mL sample of the same oxalic acid solution?

Answer 1

Balance the redox part first. You don't need to write the entire equation, just the redox part.
C2O4^2- + Cr2O7^2- ==> 2Cr^3+ + 2CO2
C in C2O4 changes from +6 (for 2C) to +8 (for 2C) which is a change of 2e.
Cr in Cr2O7^2- changes from +12 (for 2Cr) to +6(for 2 Cr) which is a change of 6e so the balanced redox part is
3C2O4^2- + Cr2O7^2- ==> 2Cr^3+ + 6CO2

millimols Cr2O7^2- = mL x M = estimated 1.3.
mmols C2O4^2- = estimated 1.3* 3 = estimated 3.9
M C2O4^2- = mmols/mL = estimated 3.9/20 = estd 0.2M

Then the acid/base titration is
H2C2O4 + 2NaOH ==> Na2C2O4 + 2H2O

mmols H2C2O4 = mL x M = 20 x 0.2 = estd 4
mmols NaOH = 2*4 = estd 8
M = mmols/mL or mL = mmols/M = estd 8/0.450 = ? mL NaOH

Answer 2

Thank you!