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Ch.3 - Molecules and Compounds
Chapter 3, Problem 48f

Write the formula for each ionic compound. f. iron(II) phosphate

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Identify the ions involved in the compound: Iron(II) indicates the iron ion is Fe^{2+}, and phosphate is the polyatomic ion PO_4^{3-}.
Determine the charges of each ion: Fe^{2+} has a charge of +2, and PO_4^{3-} has a charge of -3.
Use the criss-cross method to balance the charges: The charge of the iron ion becomes the subscript of the phosphate ion, and vice versa.
Write the formula using the subscripts obtained from the criss-cross method: Fe_3(PO_4)_2.
Ensure the formula is in its simplest form and that the subscripts are the smallest whole numbers that balance the charges.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons, resulting in the formation of positively charged cations and negatively charged anions. These oppositely charged ions attract each other, creating a stable compound. The overall charge of the compound must be neutral, which is achieved by balancing the charges of the constituent ions.
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Naming Ionic Compounds

The naming of ionic compounds involves identifying the cation and anion present in the compound. For transition metals, like iron, the oxidation state must be specified in Roman numerals, as these metals can have multiple charges. In the case of iron(II) phosphate, 'iron(II)' indicates that iron has a +2 charge, while 'phosphate' refers to the PO4^3- anion.
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Writing Chemical Formulas

To write the formula for an ionic compound, the charges of the cation and anion are used to determine the ratio of ions needed to achieve charge neutrality. For iron(II) phosphate, the iron cation (Fe^2+) and the phosphate anion (PO4^3-) combine in a 3:2 ratio, resulting in the formula Fe3(PO4)2, which reflects the need for two phosphate ions to balance the charge of three iron ions.
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