Hw3K-G1-Stoich - HW Set 3 - KEY STOICHIOMETRY CHM 2045...

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1 Note that it is possible for a compound to be ionic and contain only nonmetals. This occurs when a positive polyatomic ion such as the ammonium ion, NH 4 + , is present instead of a metal cation. HW Set 3 - KEY STOICHIOMETRY CHM 2045 HW4K-G1.STO I. COMPOUND STOICHIOMETRY Chemical Formulas 1. C Generally a substance is recognized as ionic if the constituent elements within the formula are both metal(s) and nonmetal(s) . 1 C All ionic formulas are empirical , i.e. , they represent the simplest whole-number ratio of ions present in the crystal lattice. C A substance is usually characterized as molecular if only nonmetals are present. C If the substance is molecular, the true molecular formula simply presents the actual number of atoms of each element present in the molecule. The empirical molecular formula is the simplest whole number ratio of atoms of each element present within the molecule. The molecular empirical formula may or may not represent the true molecular formula, but if a given formula of nonmetals is not in the simplest whole number ratio, the formula is then understood to be true molecular. a) LiCl b) P 4 O 10 c) P 2 O 5 d) Sc 2 S 3 metal / nonmetal nonmetals only ± ionic not simplest whole number ± ± true molecular ± molecular empirical e) PH 3 f) C 8 H 18 g) I 2 ± ± ± (also true molecular) h) N 2 H 4 i) NH 2 j) CS 2 k) H 2 O 2 ± ± ± ± l) Mg 3 (PO 4 ) 2 m) Fe(NO 3 ) 3 n) BF 3 o) S 2 Cl 2 metal / nonmetals ± ± ± ± p) NH 4 Br q) B 2 H 6 r) C H 21 NO 3 s) C H 20 N 2 O 4 but ionic! See footnote 1 ± ± ± 2. C The computation of the molar mass involves the same arithmetic as the determination of the molecular mass. Both numbers are numerically equivalent but differ in the units involved (grams versus amu's). A less precise term, "molecular weight" (MW ), is still in use and is ambiguous enough to be applied to both molecular and molar mass. C Each molar mass below will be first calculated to the maximum number of significant figures and then rounded to the number of SF's called for. a) Fe (4 SF) 55.847 ! 55.85 g/mol b) Br 2 ( 5 SF) 2 Br = 2 x 79.904 = 159.81 g/mol
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2 c) Al 2 O 3 (5 SF) d) N 2 O 4 (3 SF) 2 Al = 2 x 26.98154 = 53.96308 2 N = 14.0067 = 28.0134 3 O = 3 x 15.9994 = 47.9982 4 O = 4 x = 63.9976 101.9613 92.0110 ± ± 101.96 g/mol 92.0 g/mol e) KClO 3 f) HgBr 2 (4 SF) 1 K = 1 x 39.0983 = 1 Hg = 200.59 = 1 Cl = 35.453 = 2 Br = 79.904 = 159.808 3 O = = 360.40 122.550 g/mol ± ± 360.4 g/mol 123. g/mol g) Na 2 SO 4 142.04 g/mol h) Mg 3 (PO 4 ) 2 (6 SF) 262.858 g/mol i) C 2 H 5 OH 46.07 g/mol j) (NH 4 ) 3 AsO 4 193.03 g/mol k) CO 3 2- 60.01 g/mol l) 2+ 24.305 g/mol Note: The masses of electrons are negligible and are not a consideration when calculating molar masses of ions. m) C 12 H 22 O 11 342.30 g/mol n) FeCl 3 @ 6H 2 O 270.30 g/mol Mass to Moles and Moles to Mass 3. Note: Molar masses will be calculated to one more SF than given data. a) 0.150 mol Na x 22.99 g = 3.45 g Na b) 0.594 mol S 8 x 256.5 152 g S 8 mol 2.787 mol CH 3 Cl x 50.488 140.7 g CH 3 Cl 5.80 mol N 2 O 44.0 255 g N 2 O 48.1 mol 194.2 g 9.34 x 10 3 g K 2 CrO 4 9.0 x 10 -3 mol C 2 H 4 28.1 0.25 g C 2 H 4 0.0205 mol CS 2 76.14 g 1.56 g CS 2 0.543 mol x 61.83 33.6 g H 3 BO 3 ± i) 0.220 mol Fe 3+ 55.85 12.3 g Fe 3+ 1.00 mol S 2- 32.07 g 32.1 g S 2- 4.
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Hw3K-G1-Stoich - HW Set 3 - KEY STOICHIOMETRY CHM 2045...

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