22. London dispersion forces are attractive forces that arise due to 1. Infinite
ID: 845162 • Letter: 2
Question
22. London dispersion forces are attractive forces that arise due to
1. Infinitesimal dipoles generated by the constant random motion of electrons
2. Permanent dipoles of molecules contain covalent bonds between atoms of very different electronegatives.
3. The hydrophobic effect
4. Ion pairing between oppositely charged functional groups.
23. The aggregation of nonpolar molecules or groups in water is thermodynamically due to the
1. Increased entropy of the nonpolar molecules when they associate.
2. Decrease enthalpy of the system.
3. Increased entropy of the water molecules.
4. Very Strong van der Waals forces among the nonpolar molecules or groups.
24. The oxygen atom of water is nucleophilic because
1. It has a negative oxidation number.
2. It carries a partial positive charge.
3. It has two unshared pair of electrons.
4. It seeks electron-rich molecules.
5. All of these
25. The pH of a 10^04 M solution of HCL is
1. 3
2. 3.5
3. 4
4. 4.5
5. Greater than 4.5
26. Compare solution A with pH= 4 to solution B with pH=6.
1. The concentration of hydronium ion in solution A is twice that in solution
2. Solution A has greater buffering capacity than solution B.
3. The concentration of hydronium ion in solution A is 100 times that in solution B.
4. The hydroxide concentration are equal in two solutions since pH only measures the concentration of H+.
27. The ration of the concentration of a________ over_________ describes the proportions of forms of a weak acid necessary to satisfy the Henderson-Hasselbalch equation.
1. Conjugate acid: conjugate base
2. Conjugate base: conjugate acid
3. Proton donor; proton acceptor
4. Proton acceptor; proton donor
5. Conjugate base; conjugate acid and proton acceptor; proton donor
28. At the midpoint of a titration curve
1. The concentration of a conjugate base is equal to the concentration of a conjugate acid
2. The pH equals the pKa
3. The ability of solution to buffer is best
4. All of these
5. the concentration of a conjugate base is equal to the concentration of a conjugate acid and the pH equals to the concentration of a conjugate acid and the pH equal the pKa only
39. You have been observing an insect that defends itself from enemies by secreting a caustic liquid. Analysis of the liquid shows it to have a total concentration of formate plus formic acid (Ka = 1.8 x 10-4) of 1.45 M. The concentration of the formate ion is 0.015 M, what is the pH of the secretion?
38. Calculate the concentrations of acetic acid (pKa = 4.76) and sodium acetate required to prepare a 0.2 M buffer at pH 5.0
36. Would you expect a carboxylic acid group within the water-free interior of a protein to have a higher or lower pka than it would if it occurred on the protein
Explanation / Answer
22. 1 (Infinitesimal instantaneous dipole - dipole or dipole induced dipole interactions)
23. 4. Very strong Van Der Waal forces - AKA Hydrophobic Interactions
24. 3. pair of unshared electrons - source for nucleophilic attack
25. 3. 4 ( as pH is -log 10^-4= 4)
26. 3. 100 times more Hydronium in A. pH is a log scale and an increment of 1 means a 10 fold increase
27. 2. Conjugate base: conjugate acid
HH equation - pH=pKa + log (Conj. base)/(Conj. Acid)
28. 4. All of these (pH=pKa from above HH equation as conj base=conj acid (hint for previous question's answer as well :p ) , max buffering capacity)
39. Stay with me here, I will make this quick:
another form of HH equation: pH=pKa + log (formate)/(formic acid)
Given :- Ka = 1.8 x 10^-4 => pKa = -logka = 3.744 (Done on Calculator)
and conc of Formate is 0.015M which means conc. of Formic acid is 1.45-0.015 =1.435 M
Plugging in the HH equation to get pH = 3.744 + log (0.015)/(1.435)= 1.763 (Ans)
38. Let Conc. of Acetic acid be xM and that of Sodium Acetate be (0.2-x) M
Therefore according to HH equation - pH(5.0) = pKa(4.76) + log (0.2-x)/x
Solving for x:- log (0.2-x)/x = 0.24
therefore, (0.2-x)/x = 10^0.24 = 1.74
therefore x=0.073M (concentration of Acetic Acid)
Conc. of Sodium Acetate = 0.2-x = 0.127M
36. Beautiful Question - In highly polar or polarizable microenvironments, the charged form of an ionizable group will predominate. In less polar or polarizable microenvironments, the neutral form (COOH) will be favored and the pKa values will be shifted relative to the normal values in water [for acidic groups, the pKa values will tend to be higher (ANS) than the normal pKa values ; for basic groups, the pKa values will tend to be lower than the normal values
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