what weight of ${\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}$ (mol. wt. =126) should be dissolved in water to prepare 250 mL of centinormal solution which act as a reducing agent ?

$\mathrm{Normality} =\frac{1}{100} (\mathrm{Centinormal})$
Number of eq. = Normality x Volume (in L)
$\begin{array}{l}\mathrm{Number} \mathrm{of} \mathrm{eq}. =\frac{1}{100}\times \frac{250}{1000}=\frac{1}{400}\\ \frac{\mathrm{Mass}}{\mathrm{Molar} \mathrm{mass}}\times {\mathrm{n}}_{\mathrm{f}}\times \frac{1}{400}\end{array}$
$\begin{array}{l}\mathrm{As} \mathrm{R}.\mathrm{A}. {\mathrm{C}}_2{\mathrm{O}}_4^{2-}⟶{\mathrm{CO}}_2\\ \frac{\mathrm{w}}{126}\times 2=\frac{1}{400}\end{array}$
w =0.1575 g