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HOMEWORK PROBLEMS CHAPTER 2 (ii), CHAPTER 3 2.13 A metal is a substance that has characteristic luster, or shine, and is generally a good conductor of heat and electricity. Most metals are solids at room temperature (mercury is the sole exception). 010. The answer is c: magnesium hydroxide, Mg(OH) 2 . Errors: a) Na 2 S is sodium sulfide (typical binary compound); sodium sulfite is Na 2 SO 3 b) Calcium carbonate contains the Ca 2+ and CO 3 2- ions, so has formula: CaCO 3 . d) Ions must show the charge, so the nitrite ion is NO 2 - . NO 2 is called nitrogen dioxide (it is a foul-smelling, poisonous, brown gas) e) Iron(III) oxide contains the Fe 3+ and O 2- ions, so has formula Fe 2 O 3 . FeO must contain the Fe 2+ ion and would be called iron(II) oxide. 2.76 a. Co 3 N 2 b. (NH 4 ) 3 PO 4 c. Na 2 CO 3 d. Fe(OH) 3 Check that the total positive and negative charges in each compound are equal 2.78, 86, 122, 124. The three general types of compounds with which we are concerned are molecular (no metals); ionic with a Group I, II metal or Al; and ionic with a transition metal. Molecular compounds: use a Greek prefix to indicate subscript value (pages 65-6 of Ebbing) Ionic compounds, with Gp I, II and Al: give name of cation followed by name of anion (page 62) Ionic compound with transition metal (which can often have more than one charge for its ions): include the cation charge as a Roman numeral (page 62) 2.78 a. Na 2 O: sodium oxide (Group IA forms only 1+ cation). b. Mn 2 O 3 : manganese(III) oxide (compound of transition metal). d. Cu(NO 3 ) 2 : copper(II) nitrate (compound of transition metal). {OCL} Focus : more complete answers for (a), (b): a) Name Na2O. To do this, you must first observe that the compound is ionic since it involves a metal and a non-metal. Then, you must observe sodium, Na, is a group IA metal. Therefore, it will have a +1 charge as an ion. Thus no Roman numeral is needed. Next you must find the name for the anion and cation. Since the rule is that monatomic cations are named after the element if there is only one such ion, which is true in sodium’s case, Na+ is called the sodium ion. The concept for the names of the monatomic anions is that the name is obtained from a stem name of the element followed by the suffix -ide. Oxygen has the stem name ox- so the name of the oxygen anion would be oxide. The last concept in naming is that the name of the cation is followed by the name of the anion. Therefore, using logic, I now know the name for the compound. It would be sodium oxide.
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b) Name Mn 2 O 3 First you must observe that the element manganese, a transition metal, forms various oxidation states. To figure out the charge of manganese, you must first figure out the charge of oxygen. The charge is -2. Since there are 3 oxygens in this formula, then the compound contains a total negative charge of 3 x (-2), i.e. (-6). To counteract this charge, both atoms of Mn must hold a charge equal to +6. Since there are two manganese atoms and a charge of 6 between them, each atom must have a charge of +3.
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