Chapter 21

What is the difference between an essential element and a nonessential element?

What is the main criterion that distinguishes major, trace, and ultratrace essential elements from one another?

The concentrations of very dilute solutions are sometimes expressed as parts per million. Express the concentration of each of the following trace and ultratrace essential elements in parts per million: a. Fluorine, \(110 \mathrm{mg}\) in \(70 \mathrm{kg}\) b. Silicon, \(525 \mathrm{mg} / \mathrm{kg}\) c. Iodine, \(0.043 \mathrm{g}\) in \(100 \mathrm{kg}\)

In the human body, the concentrations of ultratrace essential elements are even lower than those of trace essential elements and therefore are sometimes expressed in parts per billion. Express the concentrations of each of the following elements in parts per billion: a. Bromine, \(6 \mathrm{mg} / \mathrm{L}\) b. Boron, \(0.014 \mathrm{g} / 100 \mathrm{kg}\) c. Selenium, \(5.0 \mathrm{mg} / 70 \mathrm{kg}\)

In the following pairs, which element is more abundant in the human body? (a) silicon or oxygen; (b) iron or oxygen; (c) carbon or aluminum

In the following pairs, which element is more abundant in the human body? (a) H or \(\mathrm{Si} ;\) (b) Ca or \(\mathrm{Fe} ;\) (c) \(\mathrm{N}\) or \(\mathrm{Cr}\)

In Chapter 2 we defined main group elements as those elements found in groups \(1,2,\) and \(13-18\) in the periodic table. Why do some chemists refer to these as the "s-block" and "p-block" elements?

Why do we classify the main group elements by group rather than by period?

Lithium oxide ( \(\mathrm{Li}_{2} \mathrm{O}\) ) and carbon monoxide (CO) have nearly the same molar mass. Why is \(\mathrm{Li}_{2} \mathrm{O}\) a solid with a high melting point, whereas \(\mathrm{CO}\) is a gas?

The nonradioactive group 17 elements are found as diatomic molecules, \(\mathrm{X}_{2}(\mathrm{X}=\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I}) .\) Why is \(\mathrm{Br}_{2}\) a liquid at room temperature, whereas \(\mathrm{Cl}_{2}\) is a gas?

Which of the following properties can be used to distinguish a metallic element from a semimetallic element: atomic radius, electrical conductivity, and/or molar mass?

Which of the following cannot be measured: ionization energy, electron affinity, ionic radius, atomic radius, or electronegativity?

\(\mathrm{PbS}, \mathrm{PbCO}_{3},\) and \(\mathrm{PbCl}(\mathrm{OH})\) have limited solubility in water. Which of them is/are more likely to dissolve in acidic solutions?

Which ion channel must accommodate the larger cation, a potassium or a sodium ion channel?

Which ion is larger: \(\mathrm{Cl}^{-}\) or \(\mathrm{I}^{-} ?\)

Place the following ions in order of increasing ionic radius: \(\mathrm{Mg}^{2+}, \mathrm{Li}^{+}, \mathrm{Al}^{3+},\) and \(\mathrm{Cl}^{-}\).

Place the following ions in order of increasing ionic radius: \(\mathrm{Br}^{-}, \mathrm{O}^{2-}, \mathrm{K}^{+},\) and \(\mathrm{Ca}^{2+}\).

Place the following elements in order of increasing electronegativity: \(\mathrm{K}, \mathrm{S}, \mathrm{F},\) and \(\mathrm{Mg}\).

Why can we estimate the electron affinity of Cl atoms by measuring the ionization energy of a \(\mathrm{Cl}^{-}\) anion?

Place the following ions in order of increasing ionization energy: \(\mathrm{Na}^{+}, \mathrm{S}^{2-}, \mathrm{F}^{+},\) and \(\mathrm{Mg}^{+}\).

Describe three ways in which ions of major essential elements (such as \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) ) enter and exit cells.

Which transport mechanism for ions requires ATP: diffusion, ion channels, or ion pumps?

Why is it difficult for ions to diffuse across cell membranes?

Why does \(\mathrm{Sr}^{2+}\) substitute for \(\mathrm{Ca}^{2+}\) in bones?

Why don't alkaline earth metal cations substitute for alkali metal cations in cases where the ionic radii are similar?

Why might nature have selected calcium carbonate over calcium sulfate as the major exoskeleton material in shells?

Bromide ion and fluoride ion are nonessential elements in the body. Do you expect their concentrations to be more similar to the concentrations of major essential elements or to the concentrations of ultratrace essential elements?

One of the functions of the alkali metal cations \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) in cells is to maintain the cells' osmotic pressure. The concentration of NaCl in red blood cells is approximately \(11 \mathrm{m} M .\) Calculate the osmotic pressure of this solution at body temperature \(\left.\left(37^{\circ} \mathrm{C}\right) . \text { (Hint: See Equation } 11.13 .\right)\)

Calculate the osmotic pressure exerted by a \(92 \mathrm{mM}\) solution of KCl in a red blood cell at body temperature \(\left.\left(37^{\circ} \mathrm{C}\right) . \text { (Hint: See Equation } 11.13 .\right)\)

Very different concentrations of \(\mathrm{Na}^{+}\) ions exist in red blood cells \((11 \mathrm{m} M)\) and the blood plasma \((160 \mathrm{mM})\) surrounding those cells. Solutions with two different concentrations separated by a membrane constitute a concentration cell. Calculate the electrochemical potential created by the unequal concentrations of \(\mathrm{Na}^{+}\).

Exoskeletons of planktonic acantharia contain strontium sulfate. Calculate the solubility in moles per liter of \(\mathrm{SrSO}_{4}\) in water at \(25^{\circ} \mathrm{C}\) given that \(K_{\mathrm{sp}}=3.44 \times 10^{-7}\).

Algae in the genus Closterium contain structures built from barium sulfate (barite). Calculate the solubility in moles per liter of \(\mathrm{BaSO}_{4}\) in water at \(25^{\circ} \mathrm{C}\) given that \(K_{\mathrm{sp}}=1.08 \times 10^{-10}\).

What danger to human health is posed by \(^{137} \operatorname{Cs}\left(t_{1 / 2} \approx 30 y \mathrm{r}\right) ?\)

Why is \(^{137} \mathrm{Cs}(t_{1 / 2} \approx 30\) yr) considered dangerous. to human health, whereas naturally occurring \(^{40} \mathrm{K}\left(t_{1 / 2}=1.28 \times 10^{6} \mathrm{yr}\right)\) is benign?

What are the likely signs of \(\Delta S\) and \(\Delta G\) for the dissolution of tooth enamel?

Why does fluorapatite resist acid better than hydroxyapatite if both are insoluble in water?

Why do superoxide ions \(\left(\mathrm{O}_{2}^{-}\right)\) act as strong oxidizing agents?

What are the products of radioactive decay of \(^{137} \mathrm{Cs} ?\) Write a balanced equation for the nuclear decay reaction.

Potassium- 40 decays by three pathways: \(\beta\) decay, positron emission, and electron capture. Write balanced equations for each of these processes.

Calculate the pH of a \(1.00 \times 10^{-3} M\) solution of selenocysteine \(\left(\mathrm{p} K_{\mathrm{a}_{1}}=2.21, \mathrm{p} K_{2_{1}}=5.43\right)\).

Calculate the \(\mathrm{pH}\) of a \(1.00 \times 10^{-3} M\) solution of cysteine \(\left(\mathrm{p} K_{\mathrm{a}_{1}}=1.7, \mathrm{p} K_{22}=8.3\right) .\) Is selenocysteine a stronger acid than cysteine?

Tooth enamel contains the mineral hydroxyapatite. Hydroxyapatite reacts with fluoride ion in toothpaste to form fluorapatite. The equilibrium constant for the reaction between hydroxyapatite and fluoride ion is \(K=8.48 .\) Write the equilibrium constant expression for the following reaction. In which direction does the equilibrium lie? $$\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH})(s)+\mathrm{F}^{-}(a q) \rightleftharpoons \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{F})(s)+\mathrm{OH}^{-}(a q)$$

Too much fluoride might lead to the formation of calcium fluoride according to the reaction $$\begin{aligned} \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH})(s)+& 10 \mathrm{F}^{-}(a q) \rightleftharpoons \\ & 5 \mathrm{CaF}_{2}(s)+3 \mathrm{PO}_{4}^{3-}(a q)+\mathrm{OH}^{-}(a q) \end{aligned}$$ Write the equilibrium constant expression for the reaction. Given the \(K_{\mathrm{sp}}\) values for the following two reactions, calculate \(K\) for the reaction between \(\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH})\) and fluoride ion that forms \(\mathrm{CaF}_{2}\). $$\begin{aligned}&\begin{array}{r} \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH})(s) \rightleftharpoons 5 \mathrm{Ca}^{2+}(a q)+3 \mathrm{PO}_{4}^{3-}(a q)+\mathrm{OH}^{-}(a q) \\\K_{3 p}=2.3 \times 10^{-59}\end{array}\\\&\mathrm{CaF}_{2}(s) \rightleftharpoons \mathrm{Ca}^{2+}(a q)+2 \mathrm{F}^{-}(a q) \quad K_{\mathrm{sp}}=3.9 \times 10^{-11}\end{aligned}$$

Tooth enamel is actually a composite material containing both hydroxyapatite and a calcium phosphate, \(\mathrm{Ca}_{8}\left(\mathrm{HPO}_{4}\right)_{2}\left(\mathrm{PO}_{4}\right)_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\left(K_{1 \mathrm{p}}=1.1 \times 10^{-47}\right)\). a. Is this calcium mineral more or less soluble than hydroxyapatite \(\left(K_{\mathrm{sp}}=2.3 \times 10^{-59}\right) ?\) b. Calculate the solubility in moles per liter of hydroxyapatite, \(\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH}), K_{4 p}=2.3 \times 10^{-59}\) in water at \(25^{\circ} \mathrm{C}\) and \(\mathrm{pH}=7.00\) c. What is the solubility of hydroxyapatite at \(\mathrm{pH}=5.00 ?\)

The \(K_{\mathrm{sp}}\) of actual tooth enamel is reported to be \(1 \times 10^{-58}\). a. Does this mean that tooth enamel is more soluble than pure hydroxyapatite \(\left(K_{\mathrm{sp}}=2.3 \times 10^{-59}\right) ?\) b. Does the measured value of \(K_{\mathrm{sp}}\) for tooth enamel support the idea that tooth enamel is a mixture of hydroxyapatite, \(\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3}(\mathrm{OH}),\) and a calcium phosphate, \(\mathrm{Ca}_{8}\left(\mathrm{HPO}_{4}\right)_{2}\left(\mathrm{PO}_{4}\right)_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) \(\left(K_{\mathrm{rp}}=1.1 \times 10^{-47}\right) ?\) c. Calculate the solubility in moles per liter of \(\mathrm{Ca}_{8}\left(\mathrm{HPO}_{4}\right)_{2}\left(\mathrm{PO}_{4}\right)_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\left(K_{\mathrm{sp}}=1.1 \times 10^{-47}\right) \mathrm{in}\) water at \(25^{\circ} \mathrm{C}\) and \(\mathrm{pH}=7.00\).

All the group 16 elements form compounds with the generic formula \(\mathrm{H}_{2} \mathrm{E}(\mathrm{E}=\mathrm{O}, \mathrm{S}, \mathrm{Se}, \text { or } \mathrm{Te}) .\) Which compound is the most polar? Which compound is the least polar?

All the group 15 elements form compounds with the generic formula \(\mathrm{H}_{3} \mathrm{E}(\mathrm{E}=\mathrm{N}, \mathrm{P}, \mathrm{As}, \mathrm{Sb}, \text { and } \mathrm{Bi}) .\) Which compound is the most polar? Which compound do you predict to have the smallest \(\mathrm{H}-\mathrm{E}-\mathrm{H}\) bond angle?

When choosing an isotope for imaging, why is it important to consider the decay mode of the isotope as well as the half-life?

Why might an \(\alpha\) emitter be a good choice for radiation therapy?

What advantage does a \(\beta\) emitter have over an \(\alpha\) emitter for imaging?