Chapter 2. Atoms, Molecules, and Ions. Solutions

Name: ___KEY______________________ Section: _____________ Chapter 2 Homeowrk: Atoms, Molecules, and Ions 1. Briefly define the following: a. Law of Conservation of Mass Matter is neither created nor destroyed in a chemical reaction – molecules change (atoms rearrange) to create new substances. b. Law of Definite Proportions All samples of a given substance will have the same ratio of atoms by mass (e.g., carbon dioxide is always CO2). c. Dalton’s Atomic Theory 4 postulates: all matter is made of atoms (once thought to be indivisible); all atoms of a given elements are identical (atoms of different elements are different); atoms are not created or destroyed in a reaction; atoms combine in small, whole-number ratios to form compounds. d. Law of Multiple Proportions Two different compounds made up of the same elements will have mass ratios that are small whole-number ratios of each other. For example, ratio of oxygen in CO to oxygen in CO2 is 1:2. 2. Describe the contribution made by the following scientists: a. Priestley Isolated oxygen by heating HgO in a closed container to observe conservation of mass. b. Thomson Worked with cathode ray tubes to discover the presence of electrons in the atom. c. Millikan Used the oil drop experiment to calculate the charge (in Coulombs) of an electron. d. Rutherford Used the gold foil experiment to determine the structure of an atom (relative positions of protons and electrons). 3. Fill in the table below: Symbol Atomic Number 1 Mass Number Number of Neutrons 1 1 0 O 8 17 9 Hg 80 199 119 Cu 29 63 34 35 71 36 1H 17 199 63 71 Br 4. How many gold-197 atoms are in a 0.156 gram sample? (Assume that the atomic mass of gold197 is very close to 197). page 1 Name: ___KEY______________________ Section: _____________ 4.77 x 1020 atoms 5. Classify the following as a mixture or pure substance, and atom, molecule, or compound. a b c d Pure substance; Molecules Mixture of atoms and compounds Pure substance; compound Pure substance; Atoms e f g h Mixture of atoms and molecules Mixture of atoms and molecules Mixture of molecules Mixture of atoms i j k l Mixture of compounds Pure substance; Atoms Mixture of atoms and molecules Pure substance; Molecules 6. Describe the difference between ionic and covalent compounds. Ionic bonds form when electrons transfer from one atom to another (between metals and nonmetals). Covalent bonds form when electrons are shared between atoms (between two or more nonmetals). 7. CH4 Circle the substances that are ionic in the list below: LiCl C2H4 NaNO3 CoCl2 CO2 KBr 8. Calculate the atomic weight for the element lithium. Lithium-6 has an natural percent abundance of 7.5% and an atomic mass of 6.015122 amu. Lithium-7 has a natural abundance of 92.5% and an atomic mass of 7.016003 amu. page 2 Name: ___KEY______________________ Section: _____________ 6.94amu 9. What is the difference between atomic mass and molar mass? Molecular mass and molar mass? Atomic mass refers to the mass of 1 atom, while molar mass refers to the mass of 6.022x1023 atoms. Molecular mass refers to the mass of 1 molecule, while molar mass refers to the mass of 6.022x1023 molecules. On both cases, the difference is 1 mol (Avogadro’s number). 10. If a C-12 atom has an atomic mass of 12 amu (exactly) and a molar mass of 12 g (exactly), how many grams are in 1 amu? 1.6605x10-24g 11. What is the difference between an element’s atomic mass number and atomic mass? An element’s atomic mass number is a whole number that denotes the number of protons and the number of neutrons. Atomic mass is not a whole number (except for Carbon-12). It is not a whole number as it usually the average of all naturally occurring isotopes. And technically, even a specific isotope’s atomic mass is not a whole number (for numerous reasons. For example, the mass of a proton and a neutron are not exactly the same. But this is beyond the scope of this class). 12. Which has a higher number of atoms, a 20.00g sample of carbon atoms or a 20.00g sample of N atoms. A 20g sample of carbon atoms. C ~ 1x1024 atoms N ~ 9x1023 atoms 13. How many atoms of He has the same mass as 1.00x1012 carbon atoms? How many atoms of Al has this same mass? 3.00x1012 He atoms (this makes sense since the mass of C ~ 3x the mass of He) 4.45x1011 Al atoms page 3