One litre of $0.15 \mathrm{M} \mathrm{Na}_2 \mathrm{SO}_3$ aqueous solution is mixed with $500 \mathrm{~mL}$…
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 number of moles of $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ remaining in the solution after the reaction?
0.1
0.0125
0.025
0.05
Solution
When $0.15 \mathrm{M} \mathrm{Na}_2 \mathrm{CO}_3$ (1 L) mixed with $0.2 \mathrm{M}$ $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ (5L) than total molarity $(M)$.
$\begin{aligned}
M & =\frac{M_1 V_1+M_2 V_2}{V_3} \\
M & =\frac{0.15 \times 1+0.2 \times 0.5}{1.5} \\
M & =0.15 \mathrm{~mol} / \mathrm{L}
\end{aligned}$
Remaining mole of $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7=0.2-0.15$
$=0.05 \mathrm{~mol} / \mathrm{L}$