Chapter 2 Water 12
Chapter 2 Water
Multiple Choice Questions 1. Weak inte interac ractio tions ns in aque aqueous ous syst systems ems Pages: 43–4 !i""iculty: 2 #ns: ! Which of these statements about hydrogen bonds is not true? true?
A) Hydrogen Hydrogen bonds bonds account account for the anomalo anomalously usly high high boiling boiling point point of water water. B) In liquid liquid water, water, the aerage water water molecule molecule forms forms hydrogen hydrogen bonds with with three to four four other water water molecules. !) Indiidual Indiidual hydrogen hydrogen bonds bonds are much much wea"er wea"er than coalent coalent bonds bonds.. #) Indiidual Indiidual hydrog hydrogen en bonds in liquid liquid water water e$ist e$ist for many seconds seconds and sometimes sometimes for minutes minutes.. %) &he strength strength of of a hydrogen hydrogen bond depend dependss on the linearit linearity y of the three three atoms inole inoled d in the bond. 2. Weak inte interac ractio tions ns in aque aqueous ous syst systems ems Page: 4$ !i""iculty: 2 #ns: # A true statement true statement about hydrophobic interactions is that they'
A) B) !) #) %)
are the driing driing force force in the formation formation of micelles micelles of amphipath amphipathic ic compounds compounds in water water. do not not contribu contribute te to the the structu structure re of water water(solu (soluble ble protei proteins. ns. hae bondi bonding ng energi energies es of appro$ appro$imate imately ly *+* *+* -oule -oule per per mole. mole. inole inole the the abilit ability y of water water to denatu denature re proteins proteins.. primarily primarily inol inolee the effect effect of polar polar solutes solutes on the the entropy entropy of aqueous aqueous systems. systems.
3. Weak inte interac ractio tions ns in aque aqueous ous syst systems ems Pages: 4$–4% !i""iculty: 2 #ns: & Hydrophobic interactions ma"e important energetic contributions to'
A) B) !) #) %)
bindin binding g of a hormon hormonee to its its recept receptor or prot protein ein.. en/yme en/yme(su (subst bstrat ratee intera interacti ctions ons.. memb membra rane ne str struc uctu ture re.. three(dime three(dimension nsional al foldi folding ng of a polypept polypeptide ide chain. chain. All All of of the the abo aboee are are true true..
4. Weak inte interac ractio tions ns in aque aqueous ous syst systems ems Page: 1 !i""iculty: 2 #ns: # #issoled solutes alter some physical 0colligatie) properties of the solent water because they change the'
A) B) !) #) %)
concen concentra tratio tion n of of the the water water.. hydrog hydrogen en bondin bonding g of of the the wate waterr. ionic ionic bondin bonding g of of the the water water.. pH of the the wat water er.. temp temper erat atur uree of the the wat water er..
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13
Chapter 2 Water
. Weak inte interac ractio tions ns in aque aqueous ous syst systems ems Page: 1 !i""iculty: 2 #ns: & 1smosis is moement of a'
A) B) !) #) %)
charged charged solute solute molecu molecule le 0ion) 0ion) across across a membrane. membrane. gas gas mole molecu cule le acro across ss a memb membra rane ne.. nonpo nonpolar lar solu solute te molec molecul ulee across across a memb membran rane. e. polar polar solu solute te molec molecule ule acro across ss a membr membrane ane.. wate waterr molec molecul ulee acros acrosss a membr membran ane. e.
'. (oni)atio (oni)ation n o" *ater+ *ater+ *eak *eak aci,s+ aci,s+ an, an, *eak *eak -ases -ases Page: 4 !i""iculty: 2 #ns: & A hydronium ion'
A) B) !) #) %)
has has the the stru struct ctur uree H213. is a hydr hydrat ated ed hyd hydro roge gen n ion. ion. is a hyd hydra rate ted d pro proto ton. n. is the usual usual form form of one one of the disso dissociati ciation on product productss of water water in solution solution.. All All of of the the abo aboee are are true true..
. (oni)atio (oni)ation n o" *ater+ *ater+ *eak *eak aci,s+ aci,s+ an, an, *eak *eak -ases -ases Page: ' !i""iculty: 2 #ns: # &he pH of a solution of 4 5 H!l is'
A) B) !) #) %)
*. *.4. 4. 4*. +4.
$. (oni)atio (oni)ation n o" *ater+ *ater+ *eak *eak aci,s+ aci,s+ an, an, *eak *eak -ases -ases Page: ' !i""iculty: 2 #ns: ! &he pH of a solution of *.4 5 6a1H is'
A) B) !) #) %)
*.4. 4.*. 4.7. 42. 4.
%. (oni)atio (oni)ation n o" *ater+ *ater+ *eak *eak aci,s+ aci,s+ an, an, *eak *eak -ases -ases Page: ' !i""iculty: 2 #ns: ! Which of the following is true about true about the properties of aqueous solutions?
A) A pH change change from from 8.* to 9.* reflect reflectss an increase increase in the hydro$ hydro$ide ide ion concen concentrati tration on 0:1H (;) of *<.
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Chapter 2 Water
14
1/. (oni)ation o" *ater+ *ater+ *eak aci,s+ aci,s+ an, *eak -ases Page: ' !i""iculty: 2 #ns: & &he pH of a sample of blood is is =., while gastric uice is pH 4.. &he blood sample has' has'
A) B) !) #) %)
*.47 *.47> > tim times es the the :H :H 3; as the gastric uice. 8.> 8.> time timess low lower er :H3; than the gastric uice. 9 tim times es lower ower :H3; than the gastric uice. 9*** 9*** time timess low lower er :H3; than the gastric uice. one one mill millio ion n tim times es lowe lowerr :H :H 3; than the gastric uice.
11. 11. (oni)ation o" *ater+ *ater+ *eak aci,s+ aci,s+ an, *eak -ases Page: !i""iculty: 1 #ns: ! &he aqueous solution with the lowest pH is'
A) B) !) #) %)
*.*4 5 H! H!l. *.4 *.4 5 acet acetic ic acid acid 0p K a .79). *.4 *.4 5 form formic ic acid acid 0p K a 2.=8). *.4 5 H!l. 4* +4 5 6a1H.
12. (oni)ation o" *ater+ *ater+ *eak aci,s+ aci,s+ an, *eak -ases Page: !i""iculty: 1 #ns: ! &he aqueous solution with the highest pH is'
A) B) !) #) %)
4 5 H!l. 4 5 6H2 0p K a >.8). *.8 5 6aH!12 0p K a 2.==). *.4 5 6a1H. *.**4 5 6a1H.
13. (oni)ation o" *ater+ *ater+ *eak aci,s+ aci,s+ an, *eak -ases Page: !i""iculty: 2 #ns: 0 @hosphoric acid is tribasic, with p K p K as of .4, 9.79, and 4.. &he ionic form that predominates at pH 2. is'
A) B) !) #) %)
H2@1. H@1 + . H@1+ . @12+ .
none of the above.
14. 0u""ering against against p changes in -iological systems Pages: % + '/ !i""iculty: 2 #ns: & Which of the following statements about buffers is true? true?
A) A buffe buffe
sed of a
" acid acid of of p K K 8 is stronger at pH than at pH 9.
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1
Chapter 2 Water
1. 0u""ering against against p changes in -iological systems Pages: %–'1 !i""iculty: 3 #ns: ! A compound has a p K p K a of =.. &o 4** m of a 4.* 5 solution of this compound com pound at pH 7.* is added 2* m of 4.* 5 hydrochloric acid. &he resulting solution is pH'
A) B) !) #) %)
9.8. 9.7. =.. =.. =.8.
1'. 0u""ering against against p changes in -iological systems Pages: '/–'1 !i""iculty: 2 #ns: & &he Henderson(Hasselbalch equation'
A) B) !) #) %)
allows allows the graphic graphic determin determinatio ation n of the molecula molecularr weight weight of a wea" acid from from its pH alone. alone. does not not e$plain e$plain the the behaior behaior of di( or tri(bas tri(basic ic wea" wea" acids. acids. emplo employs ys the the same same al alue ue for for p K a for all wea" acids. is equally equally useful useful with with solution solutionss of acetic acid acid and of hydroch hydrochloric loric acid. acid. relate relatess the the pH of a solut solution ion to the p K a and the concentrations of acid and conugate base.
1. 0u""ering against against p changes in -iological systems Pages: '/–'1 !i""iculty: 2 #ns: ! !onsider an acetate buffer, initially at the same pH as its p K p K a 0.=9). When sodium sodium hydro$ide hydro$ide 06a1H) is mi$ed with this buffer, the'
A) B) !) #) %)
pH rema remain inss cons consta tant nt.. pH rises rises more than than if an equal amount amount of 6a1H 6a1H is added added to an acetate acetate buffer buffer initial initially ly at pH 9.=9. 9.=9. pH rises rises more than than if an equal equal amount amount of 6a1H 6a1H is added added to unbuffe unbuffered red water water at pH .=9. .=9. ratio of acetic acetic acid acid to sodium sodium acetate acetate in the the buffer buffer falls falls.. sodium sodium acetate acetate formed formed precipita precipitates tes because because it it is less less soluble soluble than than acetic acetic acid. acid.
1$. 0u""ering against against p changes in -iological systems Pages: '/–'1 !i""iculty: 3 #ns: C A compound is "nown to hae a free amino group with a p K p K a of 7.7, and one other ioni/able group with a p K a between 8 and =. &o 4** m of a *. 5 solution of this compound at pH 7. was added * m of a solution of *. 5 hydrochloric hydrochloric acid. &he pH changed to 9.. &he p K p K a of the second ioni/able group is'
A) B) !) #) %)
&he pH pH cannot cannot be be determin determined ed from from this this informat information. ion. 8.. 8.9. 9.*. 9..
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Chapter 2 Water
1'
1%. 0u""ering against against p changes in -iological systems Pages: '/–'1 !i""iculty: 2 #ns: C &hree buffers are made by combining a 4 5 solution of acetic acid with a 4 5 solution of sodium acetate in the ratios shown below. below.
4 5 acetic acid Buffer 4' 4* m Buffer ' 8* m Buffer 2' >* m
4 5 sodium acetate >* m 8* m 4* m
Which of these statements is true of true of the resulting buffers? A) B) !) #) %)
pH of buf buffer fer 4 C pH pH of buffe bufferr C pH of of buffer buffer 2 pH of of buffe bufferr 4 pH of buf buffer fer pH of of buffe bufferr 2 pH of of buffe bufferr 4 D pH of buf buffer fer D pH of of buffe bufferr 2 &he proble problem m cannot cannot be soled soled witho without ut "nowing "nowing the the alue alue of of p K p K a. 6one one of of the the abo aboee
2/. 0u""ering against against p changes in -iological systems Pages: '1–'3 !i""iculty: 3 #ns: # A 4.* 5 solution of a compound with ioni/able groups 0p K 0p K as as 9. and >.8E 4** m total) has a pH of 9.7. If a biochemist adds 9* m of 4.* 5 H!l to this solution, the solution solution will change to pH'
A) B) !) #) %)
8.9*. 7.>*. >.42. >.2. &he pH cannot cannot be determ determined ined from this informati information. on.
21. 0u""ering against against p changes in -iological systems Pages: '1–'3 !i""iculty: 3 #ns: #
Fou want to maintain pH =.* =. * for an en/yme(cataly/ed reaction that will produce hydrogen ions along with the desired product. At equal concentrations, which wea" acid, if any, any, will sere as the better buffer for the reaction' reaction' Acid Acid A, A, with p-a 9.8 or Acid Acid B, with p-a =.8? A) B) !) #)
Acid A Water is as good good as either of of the acids aailable. aailable. Acid B Both Both are equall equally y effect effectie ie..
22. Water as as a reactant reactant Page: ' !i""iculty: 3 #ns: & In which reaction below does water not participate participate as a reactant 0rather than as a product)?
A) !onersion !onersion of an acid acid anhydrid anhydridee to two acids acids
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1
Chapter 2 Water
23. he "itness o" the aqueous enironment enironment "or liing organisms organisms Pages: '–'' !i""iculty: 1 #ns: & Which of the following properties of water does not contribute contribute to the fitness of the aqueous enironment for liing organisms?
A) B) !) #) %)
!ohesion !ohesion of liquid liquid water water due to hydrog hydrogen en bonding bonding High High heat heat of of apo apori/ ri/ati ation on High High spe speci cifi ficc heat heat &he density density of water water is greater greater than than the density density of ice ice &he ery ery low low molecu molecular lar weight weight of water water
hort #ns*er Questions 24. Weak interactions interactions in aqueous aqueous systems systems Pages: 4 43 3–1 !i""iculty: 2 6ame and briefly define fie types types of noncoalent interactions interactions that occur occur between biological biological molecules. #ns: 04) Hydrogen bonds' wea" electrostatic attractions between one electronegatie atom 0such as o$ygen or nitrogen) and a hydrogen atom coalently coalently lin"ed to a second electronegatie atomE 0) electrostatic interactions' relatiely wea" charge(charge interactions 0attractions of opposite charges, repulsions of li"e charges) between two ioni/ed groupsE 02) hydrophobic interactions' the forces that tend to bring two hydrophobic groups together, reducing the total area of the two groups that is e$posed to surrounding molecules of the polar solent 0water)E 0) an der Waals interactions' interactions' wea" interactions between the electric dipoles that two close(spaced atoms induce in each otherE 08) tightly bound water molecules can form as an essential part of the the binding site in a protein protein for its ligand. ligand. 2. Weak interactions interactions in aqueous aqueous systems systems Pages: 4 4' '–4$ !i""iculty: 3 %$plain the fact that ethanol 0!H 2!H1H) is more soluble in water than is ethane 0!H 2!H2).
molecules, but ethane cannot. When ethanol #ns: %thanol can form hydrogen bonds with water molecules, dissoles, the decrease in the systemGs entropy that results from formation of ordered arrays of water around the !H 2!H + group is partly compensated by the faorable interactions 0hydrogen bonds) bonds) of the hydro$yl group of ethanol with water molecules. molecules. %thane cannot form such hydrogen bonds. bonds. 2'. Weak interactions interactions in aqueous aqueous systems systems Pages: 4 4' '–4$ !i""iculty: 3 %$plain the fact that triethylammonium chloride 00!H 2!H)2 6H!l) 6H!l) is more soluble in in water than is triethylamine 00!H 2!H)2 6). 6). #ns: &he positie charge on the 6 atom in triethylammonium chloride is more polar than the uncharged 6 atom in triethylamine. triethylamine. &his increased polarity leads leads to stronger interactions with water, water, leading to increased solubility.
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Chapter 2 Water
1$
#ns: 5icelle formation minimi/es the area of the hydrophobic part of amphipathic molecules that contacts the polar solent, water. water. Hydrophobic interactions interactions between hydrophobic hydrophobic moieties are the driing force for micelle formation. When amphipathic molecules molecules form micelles in water, the entropy decrease due to the formation of ordered arrays of water molecules around the hydrophobic moieties is minimi/ed. 0ee Jig. Jig . (=, p. 7.) 2$. Weak interactions in aqueous systems Pages: 1 1–2 !i""iculty: 2 0a) Briefly define define Kisotonic,L Kisotonic,L Khypotonic,L Khypotonic,L and KhypertonicL solutions. 0b) #escribe #escribe what happens when a cell is placed in each of these types of solutions. #ns: 0a) An isotonic solution has the same osmolarity as the solution to which it is being compared. A hypotonic hypotonic solution has a lower osmolarity than the solution to which it is being compared. A hypertonic solution has a higher osmolarity osmolarity than the solution to which it is being compared. compared. 0b) Higher osmolarity results in osmotic pressure, which generally leads to moement of water across a membrane. In an isotonic solution, in which the osmolarity osmolarity of the solution is the same as the cell cytoplasm, there will be no net water moement. In a hypotonic solution, solution, water will moe into the cell, causing the cell to swell and possibly burst. burst. In a hypertonic solution, water will moe out of the cell and it will shrin". 2%. (oni)ation o" *ater+ *ater+ *eak aci,s+ an, *eak -ases Page: !i""i ""iculty: 1 Jor each of the pairs below, circle the conugate base.
M!11H
M!11 +
M6H
M6H23
H@1 +
H2@1
H!12
H!12 +
#ns: M!11 + , M6H, H@1 + , H!12 +
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1%
Chapter 2 Water
31. (oni)ation o" *ater+ *ater+ *eak aci,s+ an, *eak -ases Pages: $ $–% !i""iculty: 1 #efine p K a for a wea" acid in the following two ways' 04) in relation to its acid dissociation constant, constant, K a, and 0) by reference to a titration cure for the wea" acid. #ns: 04) p K a +log K +log K a. 0) ee Jig. (4=, p. 8>E 8>E p K a is the alue of pH at the inflection point in a plot of pH s. e$tent of titration titration of the wea" acid. At the p K p K a, the concentration of ioni/ed acid equals the concentration of un(ioni/ed acid. 32. 0u""ering against p changes in -iological -iological systems Pages: $ $–'/ !i""iculty: 2 Nie the general Henderson(Hasselbalch Henderson(Hasselbalch equation and s"etch the plot it describes 0pH against amount of 6a1H added to a wea" acid). acid). 1n your cure, label the p K a for the wea" acid and indicate the region in which the buffering capacity of the system is greatest. #ns: &he inflection point, which occurs when the wea" acid has been e$actly one half titrated with 6a1H, occurs at a pH equal to the the p K a of the wea" acid. &he region of greatest buffering capacity 0where the titration cure is flattest) occurs at pH alues of p K p K a O4. 0ee Jig. (4=, p. 8>.) 33. 0u""ering against p changes in -iological -iological systems Pages: % %–'/ !i""iculty: 3 #raw the titration cure for a wea" acid, HA, whose p K p K a is 2.. abel the a$es properly. properly. Indicate with + an arrow where on the cure the ratio of salt 0A ) to acid 0HA) is 2'4. What is the pH at this point? #ns: &he plot of pH ersus added base should hae the general shape of those shown in Jig . (4=, p. 8>, with the midpoint of the titration 0inflection 0inflection point) at pH 2.. &he ratio of A + to HA is 2 when *.=8 equialents of base hae been added. Jrom the Henderson(Hasselbalch Henderson(Hasselbalch equation, the pH at this point can be calculated' calculated'
pH p K a 3 log
:con.ugate base; :acid;
2. 3 log 2 2. 3 *.7 2.97
34. 0u""ering against p changes in -iological -iological systems Pages: ' '1 1–'2 !i""iculty: 2 What is the pH of a solution containing *. 5 acetic acid 0p K 0p K a .=) and *.4 5 sodium acetate?
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Chapter 2 Water
2/
.= + 4.7 2. 3'. 0u""ering against p changes in -iological -iological systems Pages: ' '1 1–'2 !i""iculty: 2 Jor a wea" acid with a p K p K a of 9.*, show how you would calculate the ratio of acid to salt at pH 8. #ns:
3. 0u""ering against p changes in -iological -iological systems Pages: ' '1 1–'2 !i""iculty: 3 uppose you hae ust added 4** m of a solution containing *.8 mol of acetic acid per liter to ** m of *.8 5 6a1H. 6a1H. What is the final pH? 0&he p K a of acetic acid is .=.) #ns: Addition of ** mmol of 6a1H 0** m *.8 5) to 8* mmol of acetic acid 04** m *.8 m5) completely titrates the acid so that it can no longer act as a buffer and leaes 48* mmol of 6a1H dissoled in 8** 8** m, an :1H + ; of *.2 5. Nien :1H :1H + ;, :H3; can be calculated from the water constant'
:H3;:1H + ; 4* +4 :H3; 4* +4 5 P *.2 5 pH is, by definition, definition, log 04P:H 3;). pH log 0*.2 5 P4* +4 5) 4.7. 3$. 0u""ering against p changes in -iological -iological systems
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21
Chapter 2 Water
#ns: Qse the Henderson(Hasselbalch Henderson(Hasselbalch equation to determine the ratio of ratio of :His; to :HisH 3;.
pH p K a 3 log 0:His;P :HisH 3;) =.* 9.* 3 log 0:His;P :HisH 3;) 4.* log 0:His;P :HisH 3;) :His;P :HisH 3; antilog 04) 4* &o determine the fraction the fraction of of the total in the HisH 3 form, :His; total :His; 3 :HisH 3;, fraction fraction :HisH 3;P:His;total :HisH3;P0:His; 3 :HisH 3;) :HisH3;P04*:HisH3; 3 :HisH 3;) 1511+ or /./%
substitute from ratio calculated aboe
4/. 0u""ering against p changes in -iological -iological systems Page: ' ' !i""i ""iculty: 2
H3 3 H!12(
H!12 !1 3
H 1
eere diarrhea is accompanied by a loss of H!1 2(. If untreated, will the condition result in acidosis or al"alosis? Qse the bicarbonate buffer system gien gien in the scheme aboe and e !hateliers @rinciple to e$plain your answer. Acidosis. &he remoal of H!1 2( will pull the equilibria in the direction of H!1 2(, which will #ns: Acidosis. produce H3, thereby lowering the pH. 41. Water as a reactant Page: ' ' !i""i ""iculty: 1 Nie an e$ample of a biological reaction in which water participates as a reactant and a reaction in which it participates as a product. #ns: ee page 98 for e$amples such as condensation and hydrolysis reactions. 42. he "itness o" the aqueous enironment enironment "or liing organisms organisms Pages: ' ' –'' !i""iculty: 1
If ice were denser than water, how would that aect life on earth?
