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

What weight of ferrous ammonium sulphate is needed to prepare $100 \, mL$ of $0.1 \, N$ solution? (mol. wt. $392$)

Which of the following reagents is not used in qualitative and quantitative chemical analysis?

$10 \ cm^{3}$ of $0.1 \ N$ monobasic acid requires $15 \ cm^{3}$ of sodium hydroxide solution whose normality is: (in $N$)

The required amount of crystalline oxalic acid (equivalent weight = $63$) to prepare $250 \, mL$ of $N/10$ oxalic acid solution is: (in $, g$)

$100 \ mL$ of aqueous solution of $0.05 \ M$ $Cu^{2+}$ is added to $1 \ L$ of $0.1 \ M$ $KI$ solution. The resultant solution was titrated with $0.01 \ M$ $Na_2S_2O_3$ solution using starch indicator till blue color disappeared. What is the volume (in $mL$) of $Na_2S_2O_3$ used?