Question:
When $10 \mathrm{~mL}$ of an aqueous solution of $\mathrm{Fe}^{2+}$ ions was titrated in the presence of dil $\mathrm{H}_{2} \mathrm{SO}_{4}$ using diphenylamine indicator, $15 \mathrm{~mL}$ of $0.02 \mathrm{M}$ solution of $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ was required to get the end point. The molarity of the solution containing $\mathrm{Fe}^{2+}$ ions is $\mathrm{x} \times 10^{-2} \mathrm{M}$. The value of $\mathrm{x}$ is_________
(Nearest integer)
Solution:
milli-equivalents of $\mathrm{Fe}^{2+}=$ milli-equivalents of $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$
$\mathrm{M} \times 10 \times 1=0.02 \times 15 \times 6$
$\mathrm{M}=0.18=18 \times 10^{-2} \mathrm{M}$