A 0.851 g mixture of ferrous oxalate FeC2O4 and ferric oxalate Fe2(C2O4)3 requires exactly 30.0 mL of 0.100 M KMnO4 for complete oxidation in acidic…

Permanganate takes $5$ electrons: moles $=30.0\times0.100=3.00$ mmol, so $15.0$ mmol of electrons are consumed. Per formula unit, $\mathrm{FeC_2O_4}$ loses $1$ ($\mathrm{Fe^{2+}\to Fe^{3+}}$) $+\,2$ (oxalate to $2\mathrm{CO_2}$) $=3$ electrons, while $\mathrm{Fe_2(C_2O_4)_3}$ contains only $\mathrm{Fe^{3+}}$ and loses $3\times2=6$ electrons. Let $a$ and $b$ be the millimoles of the two salts: $3a+6b=15.0$ and $143.86a+375.74b=851$ (molar masses $143.86$ and $375.74$). From the first, $a=5-2b$, so $719.3-287.72b+375.74b=851$ giving $b=1.496$ and $a=2.009$. Mass of $\mathrm{FeC_2O_4}=2.009\times143.86=289.0\ \mathrm{mg}$, which is $289.0/851\times100=33.96\%$. Check: the ferric salt contributes $1.496\times375.74=562.0$ mg, and $289.0+562.0=851$ mg, matching the sample mass.