Compounds, Naming, Reaction Equations, and Formula Weights Answers to Key Questions and Exercises

Chem Worksheet - Week 3 Compounds, Naming, Reaction Equations, and Formula Weights Answers to Key Questions and Exercises Key Questions & Exercises 1. Do all compounds contain molecules? No. Only molecular compounds contain molecules of the compound. (Ionic compounds may contain molecular ions, but these compounds are not composed of molecules corresponding to their empirical formulas.) 2. What is the difference between a molecular and empirical formula? A molecular formula represents the composition of molecules in a molecular compound. An empirical formula is the lowest whole-number ratio of elements in the compound. 3. What kinds of compounds (molecular, ionic, network) can be represented with a molecular formula? Only molecular compounds 4. What kinds of compounds (molecular, ionic, network) can be represented with an empirical formula? All three types. With ionic compounds and network solids, an empirical formula is the only appropriate type of chemical formula. 5. Using the periodic table, predict the chemical formula of the ionic compound formed by the following pairs of elements: Ga and F Ca and O Na and N Al and O GaF3 CaO Na3N Al2O3 6. Complete the following table by filling in the formula for the ionic compound formed by each pair of cations and anions, as shown for the first pair. 7. Ion K+ NH4+ Mg2+ Fe3+ S2– K2S (NH4)2S MgS Fe2S3 NO3– KNO3 NH4NO3 Mg(NO3)2 Fe(NO3)2 SO42– K2SO4 (NH4)2SO4 MgSO4 Fe2(SO4)3 PO43– K3PO4 (NH4)3PO4 Mg3(PO4)2 FePO4 Predict whether each the following compounds is molecular or ionic: PF5 - molecular, NaI - ionic, SCl2 - molecular, B2H6 - molecular, LiNO3 - ionic, NOCl - molecular, CoCO3 - ionic, NF3 - molecular 8. In the chemical formula and name, which element is given first, a metal or nonmetal? Metals are a named first, then nonmetals. 9. What suffix (ending) is added to the root of the name of the nonmetal in naming an ionic compound? The suffix -ide is added to the root; e.g., sulfur is named sulfide in an ionic compound. 10. When a metal ion can form more than one kind of cation, how is the charge on the cation indicated in the name of a compound? The charge on the cation, which can be deduced from the know charge on the anion and the number of anions in the empirical formula, is indicated by Roman numerals in parentheses; e.g., CrCl3 is chromium(III) chloride. 11. Name the following molecular compounds: SCl2, N2O4, P4O10, PF5 SCl2 - sulfur dichloride N2O4 - dinitrogen tetroxide (The “a” of tetra- is dropped before the “o” of oxide.) P4O10 - tetraphosphorous decoxide (The “a” of deca- is dropped before the “o” of oxide.) PF5 - phosphorous pentafluoride 12. Some molecular compounds are not named systematically, but rather retain their traditional names. Name the following compounds that retain their traditional names: H2O, NH3, H2O2, H2S H2O - water (not dihydrogen oxide) NH3 - ammonia (not nitrogen trihydride) H2O2 - hydrogen peroxide (not dihydrogen dioxide) H2S - hydrogen sulfide (not dihygrogen sulfide, because there is no other hydrogen compound of sulfur) 13. Name the following ionic compounds: AlCl3, Li3PO4, Ba(ClO4)2, Cu(NO3)2, Fe2(SO4)3, Ca(C2H3O2)2, Cr2(CO3)3, K2CrO4, (NH4)2SO4 AlCl3 - aluminum chloride Li3PO4 - lithium phosphate Ba(ClO4)2 - barium perchlorate Cu(NO3)2 - copper(II) nitrate Fe2(SO4)3 - iron(III) sulfate Ca(C2H3O2)2 - calcium acetate Cr2(CO3)3 - chromium(III) carbonate K2CrO4 - potassium chromate (NH4)2SO4 - ammonium sulfate 14. Give the name or chemical formula, as appropriate, for each of the following acids: HClO4, HBr, H3PO4, hypochlorous acid, iodic acid, sulfurous acid HClO4 - perchloric acid HBr - hydrobromic acid H3PO4 - phosphoric acid hypochlorous acid - HOCl (or HClO) iodic acid - HIO3 sulfurous acid - H2SO3 15. Name the following simple organic compounds: CH4 - methane, C2H6 - ethane, CH3OH methanol, C2H5OH - ethanol 16. Balance the following skeletal equations, using lowest whole-number coefficients: N2O5 + H2O ÷ 2 HNO3 3 Ca(OH)2 + 2 H3PO4 ÷ Ca3(PO4)2 + 6 H2O 2 C5H10O2 + 13 O2 ÷ 10 CO2 + 10 H2O 17. Why is it incorrect to talk about the molecular weight of NaCl? There are no molecules of NaCl. We can only define a formula weight, based on the empirical formula. 18. Would the sum of the masses of all atoms in the chemical formula C2H5OH be a molecular weight or a formula weight? This is a molecular compound, because it is composed entirely of nonmetals. Also, it is an organic compound, which we recognize as an alcohol. The given formula, then, is a molecular formula, so the sum of atomic weights multiplied by the numbers of each element would give a molecular weight. 19. Is there a difference between the molecular weight and formula weight of the molecular compound N2O5? No, because the molecular formula in this case happens to involve a lowest wholenumber ratio; viz., 2:5. Thus, the empirical and molecular formulas are identical. 20. Calculate the molecular weight and formula weight of glucose, C6H12O6. m.w. = (6)(12.01 u) + (12)(1.008 u) + (6)(16.00 u) = 180.156 u = 180.16 u empirical formula = CH2O f.w. = 12.01 u + (2)(1.008 u) + 16.00 u = 30.0267 = 30.03 u Note that in this case m.w. = 6 × f.w. As expected, the molecular weight is a whole number multiple of the formula weight. 21. Calculate the percent composition of glucose. With possibly slight differences due to rounding, the same numbers will be obtained if the empirical formula is used, instead of the molecular formula.