Question

One gram of $N{a}_{3}As{O}_4$ is boiled with excess of solid $KI$ in presence of strong $HCl$. The iodine evolved is absorbed in $KI$ solution and titrated against $0.2N$ hyposolution. Assuming the reaction to be
$As{O}_4^{3-}+2H^{+}+2I^{-}\to As{O}_3^{3-}+H_{2}O+I_2$, calculate the volume of thiosulphate hypo consumed. [Atomic weight of $As=75$ ]

• A. $48.1$ mL
• B. $38.4$mL
• C. $24.7$ mL
• D. $30.3$ mL

Answer 1

Answer: (A)

molar mass $N{a}_{3}As{O}_4$
$=23\times 3+75+16\times 4$
molar mass $=208$
$eq.ofAs{O}_4^{-}=\frac{1}{\left(\frac{208}{2}\right)}=\left(\frac{1}{104}\right)$
equivalent of $N{a}_{3}As{O}_4=$ equivalent of$I_2$
$=$ equivalent of $N{a}_2{S}_2{O}_3$
$\frac{1}{104}=.2\times V$
$\frac{1}{104\times .2}L=V=48.1ML$