Question

200 cc of $x \times 10^{-3}$ M potassium dichromate is required to oxidise 750 cc of 0.6 M Mohr's salt solution in acidic medium. Here x = ______ .

Hint:

Ensure correct n-factors: $\text{Cr}_2\text{O}_7^{2-} \to Cr^{3+}$ ($n=6$); $\text{Fe}^{2+} \to Fe^{3+}$ ($n=1$).

Answer 1

Correct Answer: 375

Use the normality equation: $N_1 V_1 = N_2 V_2$.
For $\text{K}_2\text{Cr}_2\text{O}_7$ (Oxidizing agent, n-factor $= 6$): $N_1 = 6 \times M_1 = 6x \times 10^{-3} \text{ N}$. $V_1 = 200 \text{ ml}$.
For Mohr's salt ($\text{Fe}^{2+}$ is oxidized, n-factor $= 1$): $N_2 = 1 \times M_2 = 0.6 \text{ N}$. $V_2 = 750 \text{ ml}$.
$(6x \times 10^{-3}) \times 200 = 0.6 \times 750$.
$1200 x \times 10^{-3} = 450$.
$1.2 x = 450$.
$x = \frac{450}{1.2} = 375$.