Practice for Unit IV Test / Answer Key AP Chemistry Name_____________________________ Date_________________________ Multiple Choice 1) c 2) e 3) b 4) a 5) c The shape of the curve shows that the substance was super cooled (slowly cooled to a temperature below its freezing point). The substance begins to become a solid at point Q. 6) b Both density and molarity will change due to the slight expansion of water as it is heated from 20 C to 90 C. Mole fraction will be unchanged. 7) a 8) d use the equation XA = PA/Ptotal 9) c 10) b As temperature of a gas increase, solubility of a gas decreases. As pressure above a solution increases, temperature of a gas increases. Think of what happens to the dissolved carbon dioxide in a can of cold soda, when you open the can and allow it to warm up to room temperature. 11) d 12) d 13) a 14) a 15) a 16) e 17) a 18) b 19) a 20) c 21) b 22) d Short Answer 1) Ne<CO<CH3NH2<NaF. Neon atoms are attracted to each other through London dispersion forces. The low number of electrons in the atom causes these forces to be weak. Carbon monoxide molecules are polar, in order to boil enough energy must be used to overcome both London dispersion forces and dipole forces. Methyl amine has the strongest IMF’s due to hydrogen bonding between molecules. Therefore even more energy is required to break up the IMF/s and boil the substance. Finally, in order to boil sodium fluoride, an ionic compound, enough energy must be provide to break the all of the ionic bonds holding the ions together. 2) 18.4 M H2SO4, density = 1.84 g/ml; 5.20 M H2SO4, density = 1.38 g/ml a. M1V1 = M2V2; (18.4)(V1) = (5.20)(1); V1 = 283 mL b. 18.4g 1000ml x = 1840g solution; 1ml 1L 98g 18.4molH 2 SO4 x = 1803gH 2 SO4 ; 1mol 1Lsolution x mass percent = (mass H2SO4/total mass) x 100; (1803/1840)x100 = 98% 3) letter a. Salt is spread on sidewalks to reduce the freezing point of water. Freezing point 1 depression, a colligative property, depends on the number of solute particles dissolved. When added to water NaCl dissociates into two particles; CaCl2, dissociates into three particles. Given equal moles of NaCl and CaCl2, CaCl2 will have a greater effect on the freezing point and boiling point of water because more ions are dissolved in the solution. b. Boiling occurs when the vapor pressure is equal to the atmospheric pressure. Because the vacuum pump in causing the atmospheric pressure to decrease, atmospheric pressure will equal vapor pressure at a lower temperature. So the water will boil at a lower temperature. c. Iodine a non polar compound dissolves in non polar TTE. The result is a purple solution with a density greater than the density of water. Because water is polar it does not contain any dissolved iodine and is not miscible with TTE. Because of the density differences water which is less dense is above the TTE/iodine solution. d. Perspiration involves the evaporation of water from the body. H20(l)+ heatH2O(l). This is an endothermic process because the liquid water needs heat to become a gas. The water uses heat from the body for this phase change, thus heat and water are removed from the body. e. When NaCl is added to water it dissolves and dissociates into two ions. The dissolved ions are attracted to the solvent molecules of water through ion / dipole forces. The forces of attraction between the ions and the water reduce the vapor pressure of the system. Thus it takes more energy for the vapor pressure of the system to equal the atmospheric pressure causing water to boil at a higher temperature. 4) Carbon tetrachloride is a nonpolar compound (draw the Lewis dot structure to confirm). Ammonium nitrate an ionic compound will not dissolve in polar solvent. 1-pentanol is a polar molecular compound due to the presence of the hydroxyl (OH) group at the end of the molecule. Pentane is a nonpolar compound. The nonpolar pentane will dissolve in the nonpolar carbon tetrachloride solvent, while the polar 1-pentanol will not. 5) Ionic substances dissolve if the forces of attraction between the cations and the anions are less that the force of attraction between the solvent and the ions. The forces of attraction between the highly polar water molecule and the ions are strong enough to dissociate/dissolve. The forces of attraction with the less polar ethanol are not strong enough to pull the ions apart. This can be shown by drawing the Lewis dot structures and labeling the dipoles. 6) Letter a. Oil is a nonpolar and should float on top of the water. The polar salts are dissolved in the water and must be isolated by removing the water. b. Put on your safety goggles and pour the sample into a tared beaker. Record the mass of the mixture. c. Allow the sample to settle until most of the oil appears to have risen to the top. Carefully decant the oil into a tared beaker and record the mass. Filtration cannot be used her because the viscous oil will clog the filter paper preventing both oil and the aqueous solution from passing through. d. Place the remaining liquid inot a distillation apparatus and heat gently at first to remove any volatile nonpolar material. Collect this material in a tared beaker and record its mass. e. Raise the temperature in the distillation apparatus until the water in the flask distills. Stop distillation when it appears that all of the water has distilled. The material remaining in the pot will not be completely dry. Use a wash bottle of distilled water to transfer this material to an evaporating dish. f. Heat the evaporating dish gently on a hot plate under the hood to remove any remaining water. Let cool to room temperature and record the mass of the solids. g. The mass of water can be determined by subtraction of the mass of solids, oil 2 7) 8) 9) 10) and volatile polar material from the original mass. The mass of the oil will be underestimated because it is difficult to remove all of the oil by decanting. Solid A is most likely a metal. The delocalized “sea of electrons” result in high conductivity. The strong electrostatic attraction between the delocalized electrons and the metal cations, prevents the metal from dissolving in water. Solid B is most likely ionic. Ionic solids are not conductive because their electrons not mobile – the electrons are localized around their respective nuclei. Many ionic compounds are soluble in water, and when they dissolve thy split up into anions and cations that are attracted to the polar water molecules. The separation of charges leads to formation of a conductive solution. Solid C is most likely network covalent, such as sand or diamond. The solids are not particularly conductive because the electrons are localized in the covalent bonds between atoms. They are not soluble in polar solvents because the polar molecules are covalently bonded to themselves. Solid D is most likely molecular. Molecular compounds do not conduct electricity because the electrons are localized in covalent bonds between atoms. Nonpolar molecular compounds can be insoluble or slightly soluble in water. Polar molecular compounds will be soluble in water due to intermolecular forces of attraction. The lone pairs on the nitrogen and the oxygen atoms will be attracted to the slightly positive hydrogen atoms of water, forming dipole-dipole attractions (specifically, hydrogen bonds). The nonpolar CH3 (methyl) groups, will repel the polar water molecules. The molecule may float on the water’s surface, with the hydrophobic portion facing up out of the water and the hydrophilic portion facing down. Or it may organize in layers, with the nonpolar portions facing each other, away from the water, and the polar portions facing water. Both molecules are able to make hydrogen bonds with their OH groups, so their boiling points are relatively high. Because 1-propanol’s OH group is at the end of the carbon chain, the molecule has more contact points available for London dispersion forces o attraction and its boiling point is higher. Both molecules are essentially nonpolar because of their very long carbon chains. The primary intermolecular forces of attraction are London dispersion forces. Stearic acid is a solid because it has a straight chain and so its molecules have many contact points and strong London dispersion forces overall. The double bond in oleic acid creates a kink (sp2 hybridization) in the chain so there are fewer contact points and weaker London dispersion forces. a. Sucrose, a molecular compound, does not dissociate in water while silver nitrate, an ionic compound, does. The free floating ions in the silver nitrate solution make it a good conductor. b. Solid silver nitrate does not contain any mobile charged particles so it cannot conduct electricity. Solid sodium metal atoms are held together by metallic bonding – free floating metal cations in a sea of electrons – these mobile electrons make solid sodium a good conductor. c. In the liquid phase the ions of silver nitrate can move allowing it to conduct. Sucrose does not contain charged particles in the liquid phase, so it cannot conduct. d. The ions in sulfuric acid have greater mobility when dissolved in water than in the liquid phase. The solution whas a greate conductivity than the liquid. 11) Henry’s Law: S=kP. In the closed bottle the pressure of CO 2 is 5.0 atm so the equilibrium concentration of CO2 is S = (5.0 atm) x (3.1 x 10-2 mol /L•atm). Once the bottle is open, the pressure of CO2 is significantly reduced: S = (4 x 10-4 atm) x (3.1 x 102 mol /L•atm) and soft drink becomes flat. 3