Problem L35

[Lindsay H 2B]

Sodium Bromide, NaBr, which is used to produce AgBr for use in photographic film, can be prepared as follows.
Fe + Br2 --> FeBr2
FeBr2 + Br2 --> Fe3Br8
Fe3Br8 + Na2CO3 --> NaBr + CO2 + Fe3O4
What mass of iron, in kg, is needed to produce 0.250 t of NaBr? Note that these equations must first be balanced

I looked at the solutions manual for this problem, and it shows a long sequence of dimensional analysis converting the 2.50 t NaBr to moles of Fe3Br8, then FeBr2 and then finally to Fe. My question is, why can't I just convert the number of moles straight from moles of NaBr to moles of Fe to get the mass of Fe?

[Chem_Mod]

The reason for it is that NaBr and Fe are not present together in the same chemical equation. So, to find number of moles of Fe, you need to find number of moles of FeBr2. To find number of moles of FeBr2, you need to find number of moles of Fe3Br8. You can relate only compounds that are present in the same chemical equation.

[Lauren Seidl 1D]

Yes, to approach this problem you must work backwards, using the moles of each product to find the moles of the reactant produced by the equation above it. Use the given moles of NaBr to find moles of Fe3Br8, which is the product of the previous equation above it. Use that to find the moles of FeBr2, the product produced in the first equation. Then finally, you can find the moles of Fe required, and use molar mass to convert to grams.

[Camille Ng 1D]

You need to use all the stoichiometric coefficients in the various equations to finally get a mole relationship between the two. Without the coefficients in the equation, there is no way to tell the relationship of the two in the reaction.