Iron sulfate formula unit

FIGURE E.5 Each sample contains 1 mol of formula units of an ionic compound. From left to right are 58 g of sodium chloride (NaCl), 100 g of calcium carbonate (CaCO,), 278 g of iron(ll) sulfate heptahydrate (FeS04-7H.0), and 78 g of sodium peroxide (Na. O,). 

A 2.78 g sample of hydrated iron(II) sulfate, FeS04-xH20, was heated to remove all the water of hydration. The mass of the anhydrous iron(II) sulfate was 1.52 g. Calculate the number of water molecules associated with each formula unit of FeSCU. 

Although ionic substances do not contain molecules, we can speak of the smallest unit of such a substance. The formula unit of a substance is the group of atoms or ions explicitly symbolized in the formula. For example, the formula unit of water, H2O, is the H2O molecule. The formula unit of iron(III) sulfate, Fe2(S04)3, consists of two Fe ions and three S04 ions. The formula unit is the smallest unit of such substances. 

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