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General Chemistry Principles, Patterns, and Applications, 2011

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Compounds <strong>and</strong> Covalent Molecular Substances",Table 2.4 "Common Polyatomic Ions <strong>and</strong> Their Names",<br />

<strong>and</strong> Table 2.5 "Common Cations of Metals That Form More Than One Ion". If the cation is derived from a<br />

metal that can form cations with different charges, use the appropriate roman numeral or suffix to<br />

indicate its charge.<br />

B Identify the anion using Table 2.2 "Some Common Monatomic Ions <strong>and</strong> Their Names" <strong>and</strong> Table 2.4<br />

"Common Polyatomic Ions <strong>and</strong> Their Names". Beginning with the cation, write the compound’s formula<br />

<strong>and</strong> then determine the number of cations <strong>and</strong> anions needed to achieve electrical neutrality.<br />

Solution:<br />

a. A Calcium is in group 2, so it forms only the Ca 2+ ion. B Dihydrogen phosphate is the<br />

H 2PO<br />

− ion (Table 2.4 "Common Polyatomic Ions <strong>and</strong> Their Names"). Two 4 ions are<br />

H2PO4−<br />

needed to balance the positive charge on Ca 2+ , to give Ca(H 2PO 4) 2. A hydrate of calcium<br />

dihydrogen phosphate, Ca(H 2PO 4) 2·H 2O, is the active ingredient in baking powder.<br />

b. A Aluminum, near the top of group 13 in the periodic table, forms only one cation,<br />

Al 3+ (Figure 2.11 "Metals That Form More Than One Cation <strong>and</strong> Their Locations in the<br />

2−<br />

Periodic Table"). B Sulfate is SO 4 (Table 2.4 "Common Polyatomic Ions <strong>and</strong> Their<br />

Names"). To balance the electrical charges, we need two Al 3+ cations <strong>and</strong> three<br />

SO 4<br />

2−<br />

anions, giving Al2(SO4)3. Aluminum sulfate is used to tan leather <strong>and</strong> purify drinking<br />

water.<br />

c. A Because chromium is a transition metal, it can form cations with different charges. The<br />

roman numeral tells us that the positive charge in this case is +3, so the cation is<br />

Cr 3+ . B Oxide is O 2− . Thus two cations (Cr 3+ ) <strong>and</strong> three anions (O 2− ) are required to give an<br />

electrically neutral compound, Cr 2O 3. This compound is a common green pigment that<br />

has many uses, including camouflage coatings.<br />

Cr2O3. Chromium(III) oxide (Cr 2O 3) is a common pigment in dark green paints, such as camouflage paint.<br />

Exercise<br />

Write the formula for each compound.<br />

a. barium chloride<br />

b. sodium carbonate<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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