Chapter 16.7 and 16.8Chapter 16.7 and 16.8

Acid –Base TitrationsAcid –Base Titrations

Buffered SolutionsBuffered Solutions

16.7 Acid – Base Titrations16.7 Acid – Base Titrations

Strong acids contain HStrong acids contain H++

Strong bases contain OHStrong bases contain OH--

HH+ + + OH+ OH- - H H22OO Neutralization reactionNeutralization reaction = equal amounts of = equal amounts of

H+ and OH- results in a neutral solution H+ and OH- results in a neutral solution (pH=7).(pH=7).

TitrationTitration = involves the delivery of a = involves the delivery of a measured volume of a solution of known measured volume of a solution of known concentration (concentration (titranttitrant) into the solution ) into the solution being analyzed (being analyzed (analyteanalyte))

TitrationsTitrations

Standard solutionStandard solution = solution of = solution of known concentrationknown concentration

BuretBuret = cylindrical device; allows for = cylindrical device; allows for a measured amount of liquid to be a measured amount of liquid to be dispensed.dispensed.

Stochiometry pointStochiometry point or or equivalence equivalence pointpoint =Keep adding titrant until all of =Keep adding titrant until all of analyte reacts with titrant, when pH analyte reacts with titrant, when pH = 7= 7

Titration Curve or pH curveTitration Curve or pH curve

A A titration curvetitration curve is a plot of the pH is a plot of the pH of a solution (acid or base) against of a solution (acid or base) against the volume of titrant added (base or the volume of titrant added (base or acid).acid).

Curve for the titration of a strong acid by a strong base.

Figure 17.11Figure 17.11

Titration problemsTitration problems

What volume of .1 M NaOH is needed to What volume of .1 M NaOH is needed to titrate (neutralize) 50.0 mL of .2 M HNOtitrate (neutralize) 50.0 mL of .2 M HNO33??

We need moles of OHWe need moles of OH-- added to equal added to equal moles of Hmoles of H++ present. present.

M = m / LM = m / L MMaa x V x Vaa = m = ma a and Mand Mbb x V x Vbb = m = mb b

So, MSo, Maa x V x Vaa (m (mbb/m/maa) = M) = Mbb x V x Vbb

mmbb/m/ma a (mole to mole ratio)(mole to mole ratio)

What volume of .1 M NaOH is needed What volume of .1 M NaOH is needed to titrate (neutralize) 50.0 mL of .2 M to titrate (neutralize) 50.0 mL of .2 M HNOHNO33??

NaOH + HNONaOH + HNO3 3 NaNO NaNO3 3 + HOH+ HOH MMaaVVaa (b/a) = M (b/a) = MbbVVbb

(.2 M)(50. mL)(1/1) = (.1 M)(V) (.2 M)(50. mL)(1/1) = (.1 M)(V) V=100 mL of NaOHV=100 mL of NaOH

ProblemProblem

It takes 25mL of HIt takes 25mL of H22SOSO44 to titrtate to titrtate 30mL of 3 M NaOH. Calculate the 30mL of 3 M NaOH. Calculate the molarity of Hmolarity of H22SOSO44. .

HH22SOSO4 4 ++ 22 NaOH NaOH Na Na22SOSO44 + 2 HOH + 2 HOH MMaaVVaa (b/a) = M (b/a) = MbbVVbb

MMa a (.026L) (2/1) = (3)(.030L)(.026L) (2/1) = (3)(.030L) MMa a = 1.7= 1.7

ProblemProblem

When 14.3 mL of 0.573 M Ca(OH)When 14.3 mL of 0.573 M Ca(OH)22 are neutralized with 0.426 M HNOare neutralized with 0.426 M HNO22 what volume of acid would be used?what volume of acid would be used?

Ca(OH)Ca(OH)2 2 + 2 HNO+ 2 HNO2 2 Ca(NO Ca(NO33))22 + + 2HOH2HOH

MMaaVVaa (b/a) = M (b/a) = MbbVVbb

(.426)(V) (1/2) = (.573)(14.3 mL)(.426)(V) (1/2) = (.573)(14.3 mL) V acid = 66.67 mLV acid = 66.67 mL

BuffersBuffers

Blood is a buffer. Blood’s pH ranges between 7.35 – 7.45. If the pH of blood drops below 6.8 or is higher than 7.8, you can not survive.

CO2 + H2O H2CO3 H+ + HCO3-

16.8 Buffers16.8 Buffers

A A bufferbuffer is a solution characterized is a solution characterized by the ability to resist large changes by the ability to resist large changes in pH when limited amounts of acid in pH when limited amounts of acid or base are added to it.or base are added to it.

Buffers contain either a weak acid and its conjugate base or a weak base and its conjugate acid.

Thus, a buffer contains both an acid species and a base species in equilibrium.

BuffersBuffers

Buffering capacity = the amount of Buffering capacity = the amount of H+ or OH- that a buffer system can H+ or OH- that a buffer system can absorb with out significant changes absorb with out significant changes in pH.in pH.

Aspirin is an acid. When someone Aspirin is an acid. When someone overdoses, the body can not overdoses, the body can not neutralize the aspirin.neutralize the aspirin.

BuffersBuffers

Weak acidWeak acid Weak baseWeak base saltsalt

HH22COCO33 HCOHCO33-- NaHCONaHCO33

HCHC22HH33OO22 CC22HH33OO22-- NaCNaC22HH33OO22

HCNHCN CNCN-- KCNKCN

BuffersBuffers

If you add an acid:If you add an acid:

HH++ + + HCOHCO33-- H H22COCO33

(pop) (pop) (blood) (blood)

If you add a base:If you add a base:

OHOH-- + + HH22COCO33 H H22O + O + HCOHCO33--

BuffersBuffers

Blood buffers: HBlood buffers: H22COCO33 /HCO /HCO33-- and and

HH22POPO44--/HPO/HPO44

-2-2

Kidneys: Excrete acidic/basic solutions Kidneys: Excrete acidic/basic solutions and back up the neutralization systemand back up the neutralization system

Breathing rate: controls amount of CO2 Breathing rate: controls amount of CO2 in body. COin body. CO22 + H + H22O O H H22COCO3 3

(carbonic acid)(carbonic acid) Acidosis or alkalosis – can cause death.Acidosis or alkalosis – can cause death.