Chemistry Assignment 1 Solutions

Chemistry 08 Spring 2010 Assignment 1 Solutions 1.14. Solve Freshly brewed coffee and vinegar (a, b) are homogeneous mixtures. A slice of white bread and a slice of ham (c, d) are heterogeneous mixtures. Think About It A slice of white bread is considered to be heterogeneous because its crust is different from the interior bread and the bread contains gas bubbles that are clearly seen as tiny holes in the bread. 1.18. We can distinguish between substances using either physical properties (color, melting point, density, etc.) or chemical properties (chemical reactions, corrosion, flammability, etc.). Solve We can distinguish between table salt, sand, and copper by examining their color (salt is composed of small cubic white crystals, sand is irregularly shaped and many-colored, and copper is reddish). Salt will dissolve in water while sand and copper will not. Copper conducts electricity while solid table salt and sand do not. The densities of these substances will also differ. Think About It These three substances are also very different at the atomic level. Table salt is a crystalline ionic compound composed of sodium cations and chloride anions. Sand is a compound most commonly composed of silica, a compound of silicon and oxygen. Copper is a pure element and a metal. 1.20. Physical properties are those that can be observed without transforming the substance into another substance. Chemical properties are only observed when one substance reacts with another and therefore is transformed into another substance. Solve Density, boiling point, and electrical conductivity (a, c, and d) are all physical properties while the reaction of hydrogen with oxygen (b) is a chemical property. Think About It Because the density of hydrogen gas is lower than that of any other gas, a lightweight balloon filled with hydrogen will float in air like the more familiar helium balloon. 1.30. The atomic theory is universally accepted because of the sheer volume of evidence from various experiments and observations that support the hypothesis that matter is composed of atoms. Think About It In the 1980s, the techniques of scanning tunneling microscopy and atomic force microscopy were developed to give us indirect “pictures” of atoms and molecules. Chemistry 08 Spring 2010 1.84. Collect and Organize To compare the boiling points of the two gases, we have to place the temperatures on the same temperature scale. The Kelvin scale is convenient since one of the temperatures is already in that scale and these are low-boiling gases. Analyze To convert the boiling point of O2 to Kelvin we use the equation K = ÞC + 273.15 Solve The boiling point for N2 is already in kelvins, T = 77 K. The boiling point of O2 in kelvins is K = –183ÞC + 273.15 = 90 K Think About It Because O2 has a higher boiling point, it condenses before N2 as a mixture of the two gases is cooled. 2.12. Collect and Organize In this question we use NO and NO 2 to illustrate the law of multiple proportions. Analyze The law of multiple proportions states that, if more than one compound is formed from two elements, the ratios of the masses of the two elements in each compound will be wholenumber ratios. Solve Comparing NO to NO2, we would find that the mass of oxygen present in NO 2 would be twice that found in NO for a given amount of nitrogen. Think About It Because NO and NO2 have different ratios of nitrogen and oxygen in their molecules, we expect that they will have different chemical and physical properties. 2.24. (a) Iodine’s atomic number is 53. Iodine-127 therefore has 74 neutrons and iodine-131 has 78 neutrons. Both of these isotopes have the same number of protons (they are the same element!) and iodine-131 has more neutrons of the pair. (b) Rhenium’s atomic number is 75; it has 75 protons and rhenium-188 has 113 neutrons in its nucleus. Tungsten has 74 protons and tungsten-188 will have 114 neutrons in its nucleus. Therefore, 188Re has more protons and 188W has more neutrons. (c) Nitrogen’s atomic number is 7; it has 7 protons and nitrogen-14 has 7 neutrons in its nucleus. Carbon has 6 protons and carbon-14 has 8 neutrons in its nucleus. Therefore, 14N has more protons and 14C has more neutrons. 2.26. To calculate the average atomic mass we have to consider the relative abundances according to the following formula: mx = a1m1 + a2m2 + a3m3 + where an refers to the abundance of isotope n and mn refers to the mass of isotope n. If the relative abundances are given as percentages, the value we use for an in the formula is the percentage divided by 100. Chemistry 08 Spring 2010 Solve For the average atomic mass of copper mCu = (0.6917  62.9296 amu) + (0.3083  64.9278 amu) = 63.55 amu Think About It Because copper-63 is more abundant than copper-65, we expect that the average atomic mass for copper would be below the simple average of 64.