One litre of $0.15 \mathrm{M} \mathrm{Na}_2 \mathrm{SO}_3$ aqueous solution is mixed with $500 \mathrm{~mL}$ of $0.2 \mathrm{M} \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ aqueous solution in acid medium. What is the concentration (in $\mathrm{mol} \mathrm{L}^{-1}$ ) of unreacted $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ in the resultant solution?

• A

  1. $\frac{1}{3}$
• B

  $\frac{1}{20}$
• C

  $\frac{1}{300}$
• Correct answer
  D

  $\frac{1}{30}$

Hence, the correct answer is option: 'D'.