[Rutherford]

A sample of CaCO₃ (m=0.2g) is dissoluted in HCl, the solution is then neutralized and precipitation was made with 50cm³ Na₂C₂O₄ (c=0.05mol/dm³). The precipitate is filtrated. For the titration of the acidified filtrate* 23.8cm³ of KMnO4 is spent (c=0.01mol/dm³). Calculate the % of Ca in CaCO₃.

I know the calculations and to balance the reactions but the thing that confuses me is "acidified filtrate".
After dissoluting in HCl I get CaCl₂. It reacts with Na₂C₂O₄ in excess and makes the precipitate CaC₂O₄. When I filtrate it, what is the acidifed filtrate? Is it the rest of Na₂C₂O₄ that should be acidified or CaC₂O₄? The substance that is on the filter paper or the one that went through?

[Borek]

All you are being told is that filtrate (containing unused oxalate ions) was separated from the precipitate and acidified before the titration (most likely by addition of sulfuric acid, that's the standard procedure during permanganate titration).

[Rutherford]

Ah, I thought that the filtrate is the substance that left on the paper, not the one that is separated from the precipitate. Thanks, now I got the right answer.