[badgirllili]

Hi. Thanks for viewing my post. Please help me answer this problem:

A 0.200 g sample of pyrolusite is analyzed for manganese content as follows: Add 50 mL of a 0.100 M solution of ferrous ammonium sulfate to reduce the MnO2 to Mn^2+. After reduction is complete, the excess ferrous ion is titrated in acid solution with 0.0200 M KMnO4, requiring 15.0 mL. Calculate the percent manganese in the sample as Mn3O4.

The reaction bet. Fe^2+ and MnO4^- is:
5Fe^2+ + MnO4^- + 8H^+ -> 5Fe^3+ + Mn^2+ + 4H2O

The reaction bet. Fe^2+ and MnO2 is:
2Fe^2+ + MnO2 + 4H^+ -> 2Fe^3+ + Mn^2+ + 2H2O

[Borek]

OK so far.

Hint: this is a back titration.

[badgirllili]

OK so far.

Hint: this is a back titration.

My answer is 66.74%. Did I do it right? Thanks!

[Borek]

Looks OK.