Question

What is the mass % of Chromium in sample if this sample of chromium containing alloy weighing $3.45g$ was dissolved in acid and all chromium was oxidised to $\text{CrO}_{4}^{2 -}$ . $3.15g$ of _{$Na_{2}SO_{3}$} was required to reduce the $\text{CrO}_{4}^{2 -} \text{ to } \text{CrO}_{2}^{-}$ in a basic solution while $\text{SO}_{3}^{2 -}$ ion being oxidised to $\text{SO}_{4}^{2 -}$ . (Molarmass of Cr =52)

• A. 25.13%
• B. 76.3%
• C. 80%
• D. 10%

Answer 1

Answer: (A)

Gram equivalent of Na₂SO₃ used up $= \frac{3 \text{.} 1 5}{6 3} = 5 0 \times 1 0^{- 3}$

Moles of $\text{CrO}_{4}^{2 -} = \frac{5 0 \times 1 0^{- 3}}{3} = 1 6 \text{.} 6 7 \times 1 0^{- 3}$ moles of Cr
​
Mass of Cr = 16.67 x 10^-3 x 52 = 0.867 g
% Cr is sample $= \frac{0 \text{.} 8 6 7}{3 . 4 5} \times 1 0 0 = 2 5 \text{.} 1 3 \text{\%}$