Problem 39
Duration of Hydrogen Bonds PCR is a laboratory process in which specific DNA sequences are copied and amplified manyfold. The two DNA strands, which are held together in part by hydrogen bonds between them, are heated in a buffered solution to separate the two strands, then cooled to allow them to reassociate. What do you predict about the average duration of \(\mathrm{H}\) bonds at the high temperature in comparison to the low temperature?
Problem 40
Electronegativity and Hydrogen Bonding The Pauling electronegativity is a measure of the affinity of an atom for the electron in a covalent bond. The larger the electronegativity value, the greater the affinity of the atom for an electron shared with another atom. $$ \begin{aligned} &\begin{array}{cc} \text { Abem } & \text { Electrenegativity } \\ \mathrm{H} & 2.1 \\ \mathrm{C} & 2.55 \\ \mathrm{~s} & 2.58 \\ \mathrm{~N} & 3.04 \end{array}\\\ &349 \end{aligned} $$ote that \(\mathrm{S}\) is directly beneath \(\mathrm{O}\) in the periodic table. a. Do you expect \(\mathrm{H}_{2} \mathrm{~S}\) to form hydrogen bonds with itself? With \(\mathrm{H}_{2} \mathrm{O}\) ? b. Water boils at \(100^{\circ} \mathrm{C}\). Is the boiling point for \(\mathrm{H}_{2} \mathrm{~S}\) higher or lower than for \(\mathrm{H}_{2} \mathrm{O}\) ? c. Is \(\mathrm{H}_{2} \mathrm{~S}\) a more polar solvent than \(\mathrm{H}_{2} \mathrm{O}\) ?
Problem 43
Determining Charge and Solubility of Organic Acids Suppose that, for a typical carboxyl-containing compound, the \(\mathrm{p} K_{\mathrm{a}}\) is approximately 3. Suppose \(\mathrm{HOOC}-\left(\mathrm{CH}_{2}\right)_{4}-\mathrm{COOH}, \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{4}-\mathrm{COOH}_{2}\) and \(\mathrm{HOOC}-\left(\mathrm{CH}_{2}\right)_{2}-\mathrm{COOH}\) are added to water at pH \(7 .\) a. What is the net charge of each compound in the solution? b. List the compounds in order from most soluble to least soluble.
Problem 44
Reological Effects of pH The defendant in a lawstait over industrial pollution is accused of releasing effluent of pHI 10 into a trout stream. The plaintiff has asked that the defendant reduce the eftluent's pI to no higher than 7 . The defendant's attorney, aiming to please the court, promises that his client will do even better than that: the defendant will bring the pH of the effluent down to 1! a. Will the defense attorney's suggested remecty be acceptable to the plaintiff? Why or why not? b. What facts about pH does the defense attorney need to understand?
Problem 45
Phosphate-Buffered Saline pH and Osmolarity Phosphate-buffered saline (PBS) is a solution commonly used in studies of animal tissues and cells. Its composition is 137 m? \(\mathrm{NaCl}, 2.7\) m? \(\mathrm{KCl}, 10 \mathrm{mM} \mathrm{Na}_{2} \mathrm{HPO}_{4}\left(\mathrm{pK}_{\mathrm{n}}=2.14\right), 1.8 \mathrm{~mm}\) \(\mathrm{KH}_{2} \mathrm{PO}_{4}\left(p K_{\mathrm{a}}=6.86\right)\). Calculate the pH and osmolarity of PBS. Give the osmolarity in units of osmoles per liter (osm/L).
