Which substance quickly absorbs oxygen?

$2.52 \ g$ of oxalic acid dihydrate was dissolved in $100 \ mL$ of water. $10 \ mL$ of this solution was diluted to $500 \ mL$. The normality of the final solution (in $N$) and the amount of oxalic acid (in $mg/mL$) in the solution are respectively

The weight of potassium dichromate (molecular weight $= 294$) required to prepare $250 \ mL$ of $0.04 \ N$ solution is (in $g$)

The number of equivalents of $Na_2S_2O_3$ required for the volumetric estimation of one equivalent of $Cu^{2+}$ is

The reagent commonly used to determine the hardness of water titrimetrically is

When potassium permanganate is titrated against ferrous ammonium sulphate,the equivalent weight of potassium permanganate is