Which chemical formula represents ferric ferrocyanide as listed?

Which chemical formula represents ferric ferrocyanide as listed?

• A. Fe3[Fe(CN)6]2
• B. Fe4[Fe(CN)6]2
• C. Fe4[Fe(CN)6]3
• D. Fe5[Fe(CN)6]3

Answer: (C)

Think about how the charges balance in this compound. Ferric ferrocyanide combines Fe3+ ions with the ferrocyanide unit [Fe(CN)6]4−. To make a neutral solid, the total positive charge must equal the total negative charge. Using four Fe3+ ions gives +12. Using three ferrocyanide units gives 3 × (−4) = −12. That balances to zero, so the formula Fe4[Fe(CN)6]3 fits. If you check the other formulas, they don’t balance to zero charges: they would leave a net positive or negative charge, so they don’t represent a neutral solid like ferric ferrocyanide.

Think about how the charges balance in this compound. Ferric ferrocyanide combines Fe3+ ions with the ferrocyanide unit [Fe(CN)6]4−. To make a neutral solid, the total positive charge must equal the total negative charge.

Using four Fe3+ ions gives +12. Using three ferrocyanide units gives 3 × (−4) = −12. That balances to zero, so the formula Fe4[Fe(CN)6]3 fits.

If you check the other formulas, they don’t balance to zero charges: they would leave a net positive or negative charge, so they don’t represent a neutral solid like ferric ferrocyanide.