Which substance quickly absorbs oxygen?

$K_2Cr_2O_7$ is preferred to $Na_2Cr_2O_7$ for use in volumetric analysis as a primary standard because

The primary standard solution for the estimation of $Na_2S_2O_3$ is:

$K_2Cr_2O_7$ is preferred over $Na_2Cr_2O_7$ for use in volumetric analysis as a primary standard because

$500 \ mL$ of $1.2 \ M$ $KI$ solution is mixed with $500 \ mL$ of $0.2 \ M$ $KMnO_4$ solution in basic medium. The liberated iodine was titrated with standard $0.1 \ M$ $Na_2S_2O_3$ solution in the presence of starch indicator till the blue color disappeared. The volume (in $L$) of $Na_2S_2O_3$ consumed is . . . . . . . (Nearest integer)

$20 \, mL$ of a solution of a weak monobasic acid neutralizes $22.18 \, mL$ of a solution of $NaOH$,and $20 \, mL$ of $N/10$ $HCl$ neutralizes $21.5 \, mL$ of the same $NaOH$ solution. The normality of the acid is nearly $.......$ $N$.