Question

How many grams of potassium dichromate me required to oxidise $ 20.0\, g $ of $ Fe^{2+} $ in $ FeSO_4 $ to $ Fe^{3+} $ , if the reaction is carried out in an acidic medium? Molar masses of $ K_2Cr_2O_ 7 $ and $ FeSO_4 $ are $ 294 $ and $ 152 $ respectively.

• A. $ 6.45\,g $
• B. $ 7.45\,g $
• C. $ 5.45\,g $
• D. $ 9.45\,g $

Answer 1

Answer: (A)

$K_2Cr_2O_7 + 7H_2SO_4 + 6FeSO_4 \to $
$ 3 Fe_2(SO_4)_3 + Cr_2 (SO_4)_3 + 7 H_2O + K_2SO_4$
$ = 294 .6 \times 154 = 912$
$\because 912\,g$ of $FeSO_4$ oxidises by $= 294 \,g$ of $K_2Cr_2O_7$
$\therefore 20 \,g$ of $FeSO_4$ oxidise by
$= \frac{294\times20}{6\times 154} g K_2Cr_2O_7$
$ = 6.45\,g $ of $K_2Cr_2O_7$