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| Inquiry Questions FIGURE 2.19 Buffers minimize changes in pH. Adding a base to a solution neutralizes some of the acid present, and so raises the pH. Thus, as the curve moves to the right, reflecting more and more base, it also rises to higher pH values. What a buffer does is to make the curve rise or fall very slowly over a portion of the pH scale, called the "buffering range" of that buffer. For this buffer, adding base raises pH more rapidly below pH 4 than above it; what might account for this behavior? Answer: The buffer works over a broad range because it ionizes more completely as pH increases; in essence, there is more acid to neutralize the greater amount of base you are adding. At pH4 none of the buffer is ionized. Thus below that pH, base raises the pH without the ameliorating effects of the ionization of the buffer. Self Test 1). An atom with a neutral charge must contain a). the same number of protons and neutrons. b). the same number of protons and electrons. c). more neutrons because they are neutral. d). the same number of neutrons and electrons. Answer: b 2). Electrons determine the chemical behavior of an atom because a). they interact with other atoms. b). they determine the charge of the atom. c). they can be exchanged between atoms, transferring their energy as the move. d). All of these are correct. Answer: d 3). The elements within the periodic table are organized by: a). the number of protons. b). the number of neutrons. c). the mass of protons and neutrons. d). the mass of electrons. Answer: a 4). Which of the following statements is not true. a). Molecules held together by ionic bonds are called ionic compounds. b). In NaCl, both sodium and chloride have completely filled outer energy levels of 8 electrons. c). A sodium atom is able to form an ionic bond with chloride because sodium gives up an electron and chloride gains an electron. d). Ionic bonds can form between any two atoms. Answer: d 5). Oxygen has 6 electrons in its outer energy level; therefore, a). it has a completely filled outer energy level. b). it can form one double covalent bond or two single covalent bonds. c). it does not react with any other atom. d). it has a positive charge. Answer: b 6). The atomic structure of water satisfies the octet rule by a). filling the hydrogen atoms' outer energy levels with 8 electrons each. b). having electrons shared between the two hydrogen atoms. c). having oxygen form covalent bonds with two hydrogen atoms. d). having each hydrogen atom give up an electron to the outer energy level of the oxygen atom. Answer: c 7). The partial charge separation in H2O results from a). the electrons' greater attraction to the oxygen atom. b). oxygen's higher electronegativity. c). a denser electron cloud near the oxygen atom. d). All of these are correct. Answer: d 8). The attraction of water molecules to other water molecules is a). cohesion. b). adhesion. c). capillary action. d). surface tension. Answer: a 9). What two properties of water help it to moderate changes in temperature? a). formation of hydration shells and high specific heat b). high heat of vaporization and hydrophobic exclusion c). high specific heat and high heat of vaporization d). formation of hydration shells and hydrophobic exclusion Answer: c 10). A substance with a high concentration of hydrogen ions is a). called a base. b). capable of acting as a buffer. c). called an acid. d). said to have a high pH. Answer: c Test Your Visual Understanding 1). This figure shows two energy levels (K which has an s orbital labeled 1s, and L, which has an s orbital labeled 2s and three p orbitals labeled 2p). Knowing that the lower level fills with electrons first followed by the 2s orbital and then the p orbitals, indicate the number and placement of electrons for the following elements: Carbon (C), Hydrogen (H), Fluorine (F), and Neon (Ne). Answer: Carbon: 2 electron in the 1s orbital 2 electrons in the 2s orbital 2 electrons in the 2p orbitals Hydrogen: 1 electron in the 1s orbital Fluorine: 2 electrons in the 1s orbital 2 electrons in the 2s orbital 5 electrons in the 2p orbitals Neon: 2 electrons in the 1s orbital 2 electrons in the 2s orbital 6 electrons in the 2p orbitals 2). This atom has 7 protons and 7 neutrons. What is its atomic number? What is its atomic mass? Can you predict the number of covalent bonds it will form and explain why? What is this element? Answer: The atomic number is 7, and the atomic mass is 14. This atom will form three covalent bonds. It has 5 electrons in its outer energy level and in accordance with the octet rule, it will form covalent bonds to bring the number of electrons in its outer energy level to eight. It can achieve this by forming one triple covalent bond, one double covalent bond and one single bond, or three single bonds. Any of these will fulfill the octet rule. This element is nitrogen. Apply Your Knowledge 1). The half-life of radium-226 is 1620 years. If a sample of material contains 16 milligrams of radium-226, how much will it contain in 1620 years? How much will it contain in 3240 years? How long will it take for the sample to contain 1 milligram of radium-226? Answer: After 1620 (its half-life) half of the radium-226 would be left so: 16mg/2=8mg. After 3240 (two half-lives) half of a half would be left or: 16mg/2=8mg and then 8mg/2=4mg. A half-life is 1620 years and so it would take 4 half-lives to get the sample down to 1mg or 4x1620=6480: After 1 half-life (1620 years)–16mg/2=8mg After 2 half-lives (3240 years)–8mg/2=4mg After 3 half-lives (4860 years)–4mg/2=2mg After 4 half-lives (6480 years)–2mg/2=1mg 2). Scientists find a fossil of a marine animal in the middle of a desert. They use carbon-14 dating to determine how old the fossil is, which would indicate when an ocean covered the desert. There was a 10% (0.10) ratio of 14C compared to 12C in the sample (Nf/No) with a 14C half-life (t1/2) of 5730 years. Using the equation: t = [ln(Nf/No)/(-0.693)] x t1/2 find the age of the fossil. Answer: Using the equation: t = [ln(Nf/No)/(-0.693)] x t1/2, the fossil is: t = [ln(0.10)/(-0.693)] x 5730 t = [(-2.303)/-0.693] x 5730 t = [3.323] x 5730 t = 19,041 years old An ocean covered the desert 19,041 years ago |