kristen.contreras (Offline)--90 Accepts / 102 Questions(2 Open)
1. What is the [H+] concentration in a urine sample that has a pH of 6?
A) 10-6 M pH is the negative log of the [H+] concentration
B) 10-8 M
C) 106 M
D) 10-14 M
E) 8 M
2. Why does DNA disassociate when the pH is raised above 9?
A) Protons dissociate from guanine bases disrupting the hydrogen bonding to the other strand. The proton is an “acid” and the guanine is a “base”, the combination is where the hydrogen bonding comes from. At a high pH there is a very low concentration of “protons” left to hydrogen bond with.
B) Protons bind to guanine residues giving them additional positive charges which disrupt the hydrogen bonding to the other strand.
C) Protons bind to functional groups that serve as hydrogen-bond acceptors, thus disrupting the hydrogen bonding to the other strand.
D) Protons dissociate from the phosphate groups in the backbone, which disrupts the hydrogen-bonding pattern between strands.
E) None of the above.
3. What are the primary chemical components present in a phosphate buffer at pH 7.4?
A) H3PO4 and PO4-3 D) H2PO4- and HPO4-2In a buffer the two primary components are the acid – base pair that differ by one proton. As Phosphoric acid is a multiprotic (3 H) acid there are three buffer ranges H3PO4 – H2PO4-, H2PO4- - HPO4-2, HPO4-2 – PO4-3. The first is in the acid (low pH) range. The last is in the basic (high pH) range. The middle would be in the middle (~7) pH range.
B) H2PO4- and PO4-3 E) H3PO4 and HPO4-2
C) HPO4-2 and PO4-3
4. The nucleoside adenosine exists in a protonated form with a pKa of 3.8. The percentage of the protonated form at pH 4.8 is closest to
[H+][Ad-]/[HAd] = 10-3.8 , so at pH 4.8, [Ad--]/[HAd] = 10-3.8 /10-4.8. And [Ad-]/[HAd] = 101. This means that the adenosine is almost completely unprotonated.
A) 1 B) 9 C) 50 D) 91 E) 99
T(°C) pKw
0 14.94
25 14.00
50 13.28
75 12.71
5. The ion product for liquid water, Kw, varies with temperature (T), as indicated by the change in pKw shown in the table above. The definition of neutrality is [H+] = [OH-]. Which of the following is the pH of water at neutrality at 50°C ? If [H+] = [OH-], and [H+] [OH-] = 10-13.28, then[H+]2 = 10-13.28 and [H+] = square root of 10-13.28 =10 -6.64
(A) 6.35(B) 6.64 (C) 7.00 (D) 7.40 (E) 13.28
6. What is the pH of 0.0025 M HCl?
[H+] = 2.5 x 10-3 means that the pH is 3 - .39794 ~ 2.6
7. What is the pH of 0.0025 M lactic acid. The pKa of lactic acid is 3.86. Show all work.
Ka = 10-3.86 = [H+][A-]/[HA]. 10-3.86= [H+][A-]/[2.5 x 10-3 - dissociated]. Since both [H+] and [A-] come from the dissociation of lactic acid (we are assuming the contribution from dissociation of water is negligible)10-3.86= [H+]2/[2.5 x 10-3 - [H+]] solving for [H+] is difficult unless we assume that the [H+] is negligible relative to 2.5 x 10-3. [H+] = 10– 5.75 and pH is 5.75.
8. What is the pH of a buffer made of 0.0025 M sodium lactate and 0.004 M lactic acid? Show all work. The two compounds are an acid and conjugate base pair. pKa for lactic acid is 3.86 from above. 10-3.86 = [H+][A-]/[HA] = [H+][10-2.6]/([4x10-3] – dissociated [H+]). Assuming that the dissociated acid is negligible, [H+] = 10-7.86 . This is negligible relative to4x10-3 so pH = 7.86.
9. What is the pH of the buffer in 8 if 0.001 M HCl is added to the buffer solution? Show all work. 10-3.86 = [H+][A-]/[HA] = [10-3-x][10-2.6-x]/([4x10-3] + x (asssociated acid)). This will actually drive some of the sodium lactate to the associated acid. X is not insignificant so this will be solving a quadratic equation.
10. What is the pH of the buffer in 8 if 0.001 M NaOH is added to the buffer solution? Show all work. . 10-3.86 = [H+][A-]/[HA] = [x][10-2.6+x]/([4x10-3] - 10-3 (disssociated acid)). This will actually drive some more of the lactic acid to the dissociated acid. X is not insignificant so this will be solving a quadratic equation.
11. What is the ratio of pyruvate ion to pyruvic acid at pH 4.86? The pKa of pyruvic acid is 2.50. 10-2.50 = [H+][A-]/[HA] = [10-4.86][A-]/([HA] – [A-])
12. At what pH does pyruvic acid have the greatest buffering capacity? At pKA 2.50 the [H+] = [OH-]. This pH will give you the greatest buffering power.