Vermont Tech Chapter 2 Practice Problem Key These examples can be used with tutors or during help sessions. Atomic Theory of Matter 1. A chemist finds that 30.82 g of nitrogen will react with 17.60 g, 35.20 g, 70.40 g or 88.0 g of oxygen to form four different compounds. a. Calculate the mass of oxygen for each gram of nitrogen in each compound. b. How do the results of your calculations for part a) support Dalton s atomic theory? a. O/N = 17.60/30.82 = 0.57 / 0.57 = 1 O/N = 35.20/30.82 = 1.14 /0.57 = 2 O/N = 70.40/30.82 = 2.28 / 0.57 = 4 O/N = 88.0/30.82 = 2.85 / 0.57 = 5 b. The ratio of atoms in a molecule is described by a small whole number Discovery of Atomic Structure 2. A charged particle is shot through two electrically charged plates as shown below. a. Why does the particle s path bend? b. What is the charge of the particle, + or -? c. If the charge on the plates is increased will the degree of bending change? If so, how? d. If the mass of the particle is increased but the speed is unchanged, will the degree of bending stay the same, increase or decrease? a. The particle s path bends because it is charged. b. Since the particle is attracted to the positively charge plate, the particle is negatively charged. c. Increasing the charge will increase the bend towards the + plate or away from the plate. d. If the mass of the particle is changed but it s speed and charge stay the same, bending will decrease because momentum will carry the particle forward. 1
Modern View of Atomic Structure 3. The diameter of a dime is 17.9 mm. The diameter of an Ag atom is 2.88 Å. How many silver atoms can fit across the diameter of a dime? 17.9 mm 1 m 10 10 Å 1 Ag atom = 6.22 x 10 7 atoms/dime 1 dime 1000 mm 1 m 2.88 Å 4. A cube of gold that is 1.00 cm on each side has a mass of 19.3 g. A single gold atom has a mass of 197.0 amu. a. If 5.90 x 1022 atoms of gold are in the cube, calculate the diameter of one gold atom. b. What assumptions did you make to answer b? a. cube root of 5.9x10 22 = 3.89x10 7 atoms per side 1 cm 1 m 10 10 Å = 2.57 Å/Au atom 3.89x10 7 atoms 100 cm 1 m b. Assumed that atoms were identical and even spheres distributed perfectly throughout the cube. Atomic Numbers, Mass Numbers & Isotopes 5. How many protons, neutrons & electrons are there in these atoms: a. 40 Ar 18 p, 18e, 22 n b. 65 Zn 30 p, 30 e, 35 n c. 70 Ga 31 p, 31 e, 39 n 6. How many protons, neutrons & electrons are there in a 138 Ba atom? Ba is atomic number 56, so 56 protons & electrons. Number of neutrons = 138 56 = 82 neutrons. 7. How many protons, neutrons & electrons in an atom of 31 P? P is atomic number 15, so 15 protons & electrons. Number of neutrons = 31 15 = 16 neutrons. 2
8. Magnesium has three isotopes with mass numbers 24, 25 and 26. a. Write the chemical symbol (with super- and subscripts) for each isotope. b. How many neutrons are there in each of those isotopes? a. 24 12Mg, 25 12Mg, 26 12Mg b. 12, 13, 14 neutrons respectively Atomic Weight 9. A particular atom of chromium has a mass of 52.94 amu. But the atomic weight of chromium is 51.99. How can that be? What s the explanation? 51.99 is the average atomic mass. The atom with a mass of 52.94 is one particular type of chromium isotope. 10. Three isotopes of silicon occur in nature: 28 Si (92.23%) with an atomic mass of 27.97693amu; 29 Si (4.68%) with an atomic mass of 28.97649 amu; and 30 Si (3.09%) with an atomic mass of 29.97377 amu. Calculate the average atomic mass of silicon. Avg amu = (0.9223)(27.97693) + (0.0468)(28.97649) + (0.0309)(29.97693) = 28.09 amu The Periodic Table 11. Match these elements with their descriptions: Ar, H, Ga, Al, Ca, Br, Ge, K, O a. alkalai metal K b. alkaline earth metal Ca c. noble gas Ar d. halogen Br e. metalloid Ge f. nonmetal in group IA H g. metal whose ion has a +3 charge Al h. nonmetal whose ion has a - 2 charge O i. element that resembles Al Ga Molecules & Chemical Formulas 12. How many? a. carbon atoms in C2H5COOCH3 4 b. oxygen atoms in Ca(ClO4)2 8 c. hydrogen atoms in (NH4)2(HPO4) 9 3
13. Write the empirical formulas of these molecular compounds: a. Al2Br6 AlBr3 b. C8H10 C4H5 c. C4H8O2 C2H4O Ions & Charges 14. A particle has 16 protons, 16 neutrons and 18 electrons. Is it an atom, a cation or an anion? With two extra electrons the particle is an anion. 15. What are the charges of the most stable ions formed by these elements: a. Mg +2 b. Al +3 c. F - 1 Ionic Compounds 16. What are the charges of the metal ions in each of these compounds? a. NiO +2 b. MnO2 +4 c. Cr2O3 +3 Naming Compounds 17. Label each compound as molecular or ionic? a. PF3 molecular b. NaI ionic c. SCl2 molecular Ionic Naming 18. Give the names and charges of the cation and anion in each of these ionic compounds: a. CaO calcium +2, oxide - 2 b. Ag2(SO4) silver +1, sulfate - 2 c. Al(ClO3)3 aluminum +3, chlorate - 1 Molecular Naming 19. Give the name or formula, as appropriate. a. SF6 sulfur hexafluoride b. IF5 iodine pentafluoride c. tetraphosphorous hexasulfide P4S6 4
20. The oxides of nitrogen are very important components of urban air pollution. Name each: a. N2O dinitrogen monoxide b. NO nitrogen monoxide c. NO2 nitrogen dioxide Naming Acids 21. Give the names or formulas of these acids, as appropriate: a. H(BrO3) bromic acid b. H3(PO4) phosphoric acid c. hypochlorous acid H(ClO) Naming Reviews 22. Gives the names of these common chemicals: a. NaCl, table salt sodium chloride b. Na(HCO3), baking soda sodium hydrogen carbonate c. Na(ClO), in bleaching products sodium hypochlorite 23. Many names and formulas are easily confused. So try writing formulas or names for each of these pairs: a. calcium sulfide vs. calcium hydrogen sulfide CaS vs. Ca(HS)2 b. hydrobromic acid vs. bromic acid HBr vs. HBrO3 c. aluminum nitride vs. aluminum nitrite AlN vs. Al(NO2)3 5