Ferrous oxide (FeO) crystal has a cubic structure and each edge of the unit cell is $$5.0$$ Ao. Taking density of the oxide as $$4.0$$ g $$cm-3$$. The number of FeO formula units present in each unit cell is:

Question

Ferrous oxide (FeO) crystal has a cubic structure and each edge of the unit cell is $$5.0$$ Ao. Taking density of the oxide as $$4.0$$ g $$cm-3$$. The number of FeO formula units present in each unit cell is:

Infinity Learn · Accepted Answer

Volume of the unit cell, $$a3=5$$×$$10$$−$$83=1.25$$×$$10$$−$$22cm3$$ $$($$∵$$a=5$$Å $$)$$  Density $$($$ρ$$)=4.0gcm$$−$$3$$ Mass of unit cell $$=V$$×ρ$$=1.25$$×$$10$$−$$22$$×$$4.0=5.0$$×$$10$$−$$22g$$ Mass of one molecule of $$FeO=$$ Mol. mass (in$$ $$grams)  Avogadro's number $$=726.022$$×$$1023=1.195$$×$$10$$−$$22g$$ Number of FeO molecules per unit cell $$=5.0$$×$$10$$−$$22g1.195$$×$$10$$−$$22g=4.19$$≈$$4Thus$$, there are four FeO formula units in each unit cell.

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