Exam I Practice

Exam I Practice - 1 Which statement is incorrect about the...

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1 Which statement is incorrect about the nature of the hydrogen bond? a. The donor is a hydrogen atom bonded to a less electronegative atom than hydrogen. b. The more linear the bond, the stronger the interaction. c. The acceptor is a fairly electronegative atom containing a nonbonding pair of electrons. d. It is a type of noncovalent bond. ANSWER A. 2 If equal molar amounts of Na 2 HPO 4 and Na 3 PO 4 are mixed in water, calculate the resulting pH. The pKas of phosphoric acid are 2.1, 7.2, 12.4. a. 7.0 b. 12.4 c. 4.5 d. 9.5 pH = pKa3 + Log PO4/HPO4, Log PO$/HPO4 = 0 , so pH = pKa3 = 12.4 3 Which of the following characteristics is not associated with the solvent water? a. high surface tension b. a chemically inert solvent, which has a great capability to dissolve a diverse spectrum of molecules and ions c. low heat capacity d. a high dielectric constant ANSWER C 4. Which of the following amino acids is the least water soluble at pH 7.0? a. tryptophan b. glutamic acid c. cysteine d. histidine A. Trp is aromatic and nonpolar, nonpolar does not dissolve in polar 5. Calculate the percent of cysteine molecules which will have the sulfhydryl group hydrogen dissociated at pH 7.5. The pKa for the cysteine side chain is 8.3. a. 6.5% b. 21.0% c. 43.0% d. 13.7% For 5, You want % of SH in the form of S- Again, pH = pKa + Log A-/HA and pKA = 8.3 , You find the S-/SH ( same as A-/HA) ratio and from there use the fact that
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S- + SH = 100% = Total Cysteine. S-/SH = antilog (pH-Pka) = anti log (7.5-8.3) = 0.158 , and S- = 0.158 SH or SH = S-/0.158
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Exam I Practice - 1 Which statement is incorrect about the...

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