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Multimedia Chemistry 1 & 2
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Multimedia Chemistry I & II (1996-9-11) [English].img
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chapt11.2c
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à 11.2cèpH, pOH, Strong Acids å Strong Bases
äèPlease fïd ê specified quantity ï êse solutions ç strong acids å strong bases.
âèWhat are ê pH å pOH ç a 0.00280 M HNO╕ solution?èpH is
defïed as -log[Hó], å pOH is defïed as -log[OHú].èHNO╕ is a strong
acid, which means that it completely dissociates ï water.èA 0.00280 M
HNO╕ solution actually exists as 0.00280 M Hó å 0.00280 M NO╕ú.èThe
pH = -log(0.00280) = 2.55.èWe can use pH + pOH = 14.00 ë fïd ê pOH.
pOH = 14.00 - pH.èpOH = 14.00 - 2.55 = 11.45.
éSèWe can tell from electrical conductivity studies that water dis-
sociates slightly accordïg ë ê equilibrium reaction,
2H╖O = H╕Oó + OHú.
The equilibrium constant expression for this equilibrium is
èè [H╕Oó][OHú]
K╦ = ───────────.
èèè [H╖O]ì
Water has a molarity ç 55.3 M at 25°C.èThe concentrations ç H╕Oó å
OHú must be very small ï pure water because pure water does not conduct
electricity ë any significant extent.èSïce ê extent ç dissociation
is so slight, ê water concentration is essentially constant ï ê
above equilibrium.èThe water concentration can be ïcluded with K╦ ë
defïe a new equilibrium constant, K╨, called ê ion product ç water.
K╦[H½O]ì = K╨
K╨ = [H╕Oó][OHú] = 1.00x10úîÅ at 25°C.
In pure water, [H╕Oó] = [OHú]; å [H╕Oó]ì = 1.00x10úîÅ.èTakïg ê
square root:èèèèèèèèèèèè
[H╕Oó] = 1.00x10úÆ M.
The percentage dissociation ç water is (1.00x10úÆ M/55.3 M)x100, which
is 1.8x10úÆ%.èTo gaï an appreciation for this small percentage consider
ê followïg.èThe world population ï 1992 was 5.5 billion people.èIf
people were water molecules, only about 10 people ï ê world would have
split ïë two parts.
èèIn neutral solutions, [H╕Oó] = [OHú] = 1.00x10úÆ M at 25°C.
Acidic solutions have a higher concentration ç H╕Oó:
èèèèè[H╕Oó] > 1.00x10úÆ M, å [OHú] < 1.00x10úÆ M.
Basic solutions have a higher concentration ç OHú:
èèèèè[H╕Oó] < 1.00x10úÆ M, å [OHú] > 1.00x10úÆ M.
In many chemical processes ïvolvïg acids å bases, ê H╕Oó å OHú
concentrations vary over a wide range.èSorenson, a Danish biochemist,
ïtroduced a logarithmic scale ë compress ê range ç values ç ê
concentrations.èThe pH scale is defïed as
pH = -log([H╕Oó]).
Takïg ê log ç a number fïds ê power ç 10 that yields ê number.
log(100) = 2.èStartïg with [H╕Oó][OHú] = 1.00x10úîÅ å takïg ê
-log gives: -log([H╕Oó]) + (-log([OHú])) = -log(1.00x10úîÅ)
èèèèèèèèèèèpH + pOH = 14.00.
The previous equation is useful because sometimes it is easier when usïg
your calculaër ë fïd ê pOH first.
èèNeutral solutions have a pH ç 7.00 at 25°C.
èèAcidic solutions have pH values less than 7.00.
èèBasic solutions have pH values greater than 7.00.
If we know ê pH (or pOH), we can fïd ê [H╕Oó] by takïg ê antilog.
[H╕Oó] = antilog(pH) = ╢╡-pH
What is ê pH ç a 0.020 M HCl solution?èHydrochloric acid is a strong
acid which means that it dissociates essentially 100%.è0.020 M HCl is
actually 0.020 M H╕Oó å 0.020 M Clú.èTherefore, pH = -log(0.020).
pH = 1.70.èThe pH is less than 7.00, as we expect for an acidic solution.
What is ê pH ç a 0.015 M NaOH solution?èSodium hydroxide is a strong
base which means that it also dissociates 100%.è0.015 M NaOH is
0.015 M Naó å 0.015 M OHú.èWe can fïd ê pOH å ên ê pH.
pOH = -log(0.015) = 1.82.èpH = 14.00 - pOH.èpH = 14.00 - 1.82 = 12.18.
The pH ç ê NaOH solution is 12.18.èWe expect ê pH ë be above 7.00
for a basic solution.
What is ê pH ç a solution obtaïed by addïg 5.0 mL ç 6.0 M HCl ë
95.0 mL ç water?èWe need ë know ê H╕Oó concentration ï order ë
calculate ê pH.èHCl is a strong acid, so we thïk ç a 6.0 M HCl solu-
tion as beïg 6.0 M H╕Oó å 6.0 M Clú.èMixïg ê acid with water
dilutes ê acid.èWe use ê equation: M╢V╢ = M╖V╖ ë fïd ê new con-
centration ç H╕Oó.èM╖ = M╢∙V╢/V╖.èAssumïg that ê volumes can be
added, we getèèè ?M╖ = (6.0 M)(5.0 mL)/(5.0 + 95.0 ml).
èèèèèèèè [H╕Oó] = 0.30 M.èpH = -log(0.30).èpH = 0.52.
The diluted solution should have a pH ç 0.52.
1èWhat is ê [H╕Oó] ï a 1.12x10úÄ M HCl solution?èHCl is a
èèèèè strong acid.
è A) 0.0335 M B) 5.60x10úÅ M
è C) 1.12x10úÄ M D) 8.9x10úÉ M
üèSïce HCl is a strong acid, we know that it completely dissoci-
ates ï an aqueous solution.èA 1.12x10úÄ M HCl solution is actually
1.12x10úÄ M H╕Oó å 1.12x10úÄ M Clú.
Ç C
2èWhat is ê [OHú] ï a 3.60x10úì M HCl solution?èHCl is a
èèèèè strong acid.
è A) 0.0360 M B) 2.8x10úîÄ M
è C) 5.3x10úÆ M D) 1.00x10úÆ M
üèSïce HCl is a strong acid, we know that it completely dissoci-
ates ï an aqueous solution.èA 3.60x10úì M HCl solution is actually
3.60x10úì M H╕Oó å 3.60x10úì M Clú.èIn an aqueous solution ê H╕Oó
å OHú concentrations are always ïterrelated through ê ion product ç
water.èKw = [H╕Oó][OHú] = 1.00x10úîÅ.èTherefore, [OHú] = K╨/[H╕Oó].
èèèèèèèèè [OHú] = (1.00x10úîÅ)/(3.60x10úì) = 2.8x10úîÄ M.
Sïce ê solution is acidic we know that [OHú] must be less than 1x10úÆ.
Ç B
3èWhat is ê pH ç a 6.00x10úÄ M HNO╕ solution?èNitric acid is
è a strong acid.
è A) 2.22 B) 1.11
è C) 0.0060 D) 3.60
üèSïce nitric acid is a strong acid, 6.00x10úÄ M HNO╕ is viewed as
beïg 6.00x10úÄ M H╕Oó å 6.00x10úÄ M NO╕ú.èThe pH equals -log([H╕Oó]).
pH = -log(6.00x10úÄ).èpH = 2.22.
Ç A
4èWhat is ê pH ç a 0.015 M KOH solution?èPotassium hydroxide
èèèèè is a strong base.
è A) 1.82 B) 7.91
è C) 6.09 D) 12.18
üèSïce KOH is a strong base, we know that 0.015 M KOH is 0.015 M
Kó å 0.015 M OHú.èWe can easily fïd ê pOH å ên ê pH.
pOH = - log([OHú]).èpOH = -log(0.015) = 1.82èBasic solutions have a pH
value above 7.00.èpH = 14.00 - pOH.èpHè= 14.00 - 1.82 = 12.18.è
Ç D
5èA sample ç sea water has a pH ç 8.3.èWhat is ê [H╕Oó] ï
èèèèè ê sample?
è A) 3x10úô M B) 5x10úö M
è C) 2x10úæ M D) 3x10úÆ M
üèpH is defïed as -log([H╕Oó]).èTo fïd ê [H╕Oó] from ê pH,
we must simply take ê antilog.èThis uses ê "INV" or "2nd function"
key on your calculaër.
èèèèèèèèèèèè [H╕Oó] = ╢╡-pH = ╢╡-8.3è= 5x10úö M.
Ç B
6èWhich 0.10 M solution is ê most acidic?èThe pH values ç
èèèèè ê solutions are ï parenêsis.
è A) H╕PO╣ (1.5) B) HC╖H╕O╖ (2.9)
è C) NH╣Cl (4.6) D) Na╖CO╕ (11.6)
ü The most acidic solution has ê lowest pH value, because ê pH
is ê negative logarithm ç ê Hó concentration.èThe solution with ê
lowest pH is ê H╕PO╣ solution, so it is ê most acidic ç ê choices
ï ê problem.
Ç A
7èWhich 0.10 M solution is ê most basic?èThe pH values ç ê
èèèèè solutions are ï parenêsis.
è A) H╖SO╣ (1.2) B) Na╖SO╣ (6.1)
è C) NaC╖H╕O╖ (8.4) D) Na╕PO╣ (11.6)
üèThe most basic solution has ê highest pH value, because ê pH
is ê negative logarithm ç ê Hó concentration.èThe solution with ê
highest pH is ê Na╕PO╣ solution, so it is ê most basic ç ê choices
ï ê problem.
Ç D
8èWhat is ê pH ç solution that is prepared by addïg 25.0 mL
ç 0.50 M HBr ë 525 mL ç water?èAssume that ê volumes are additive.
è A) 0.30 B) 1.90
è C) 3.02 D) 1.64
üèAddïg ê HBr solution ë ê water decreases ê [H╕Oó] ï ê
HBr solution.èWe need ë fïd ê [H╕Oó] ï ê diluted HBr solution.
You can use ê equation M╢V╢ = M╖V╖ for ê dilution, because HBr is ê
only source ç H╕Oó excludïg water.èThe [H╕Oó] from ê water is ïsig-
nificant.
[H╕Oó] = (0.50 M H╕Oó)(25.0 mL)/(525 mL + 25 mL) = 0.023 M H╕Oó.
The pH ç ê diluted solution is:èpH = -log(0.023) = 1.64.
Ç D
9èWhat is ê pH ç solution that is prepared by addïg 5.0 mL
ç 6.0 M NaOH ë 245 mL ç water?èAssume that ê volumes are additive.
è A) 1.52 B) 0.91
è C) 13.08 D) 12.48
üèAddïg ê NaOH solution ë ê water decreases ê [OHú] ï ê
NaOH solution.èWe need ë fïd ê [OHú] ï ê diluted NaOH solution.
For a dilution, M╢∙V╢ = M╖∙V╖.èM╖ = M╢∙V╢/V╖.
?[OHú] = (6.0 M OHú)(5.0 mL)/(245 mL + 5 mL) = 0.12 M OHú.èThe pOH ç
ê diluted solution is:èpOH = -log(0.12) = 0.92.èThe pH ç ê solu-
tion is pH = 14.00 - pOH.èpH = 14.00 - 0.92.èpH = 13.08.
Ç C
10èA LiOH solution had a pH ç 11.48.èWhat was ê hydroxide
èèèèèèion concentration ï ê solution?
èèA) 9.1x10úæ M B) 3.0x10úÄ M
èèC) 3.3x10úîì M D) 5.5x10úì M
üèThere are two ways ë fïd ê hydroxide ion concentration.èFrom
ê pH, we can fïd ê [H╕Oó] å ên fïd [OHú].èAlternatively, we
could fïd pOH from pH å ên fïd [OHú].èpOH = 14.00 - pH.
pOH = 14.00 - 11.48 = 2.52.è[OHú] = ╢╡-pOH = ╢╡-2.52 = 3.0x10úÄ M.
The hydroxide ion concentration ï ê LiOH solution is 3.0x10úÄ M.èYou
should expect a number greater than 1x10úÆ M sïce ê solution is basic.
Ç B