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  4. In the standard is ation of N a2 S2 O3 using K2 C r2 O7 by iodometry, the equivalent weight of K2 Cr 2 O 7 is

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Q. In the standard is ation of $N a_{2} S_{2} O_{3}$ using $K_{2} C r_{2} O_{7}$ by iodometry, the equivalent weight of $K_{2} Cr _{2} O _{7}$ is

Chhattisgarh PMTChhattisgarh PMT 2009

Solution:

$Cr _{2} O _{7}^{2-}+14 H ^{+}+6 e^{-} \rightarrow 2 Cr ^{3+}+7 H _{2} O$
$6 I ^{-} \rightarrow 3 I _{2}+6 e^{-} $
$2 Na _{2} S _{2} O _{3}+ I _{2} \rightarrow Na _{2} S _{4} O _{6}+2 NaI$
In this reaction, Eq. wt. of $K _{2} Cr _{2} O _{7}=\frac{\text { Molecular weight }}{3 \times 2}$
$=\frac{\text { Molecular weight }}{6}$