titration of vineg1ar.pdf

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Chemistry lab ses s ion n 7 :

T i t rat ion o f v inegar

Pu rpo s e s : - prepare a diluted solution ;

-carry out of a coloured titration ;

-carry out of a conductimetric titration ;

-carry out of a pHmetric titration ;

-determine the end point of a titration ;

-compute the concentration of ethanoic acid in vinegar ;

-compare different techniques of titration

! " P r e p a r a t i o n o f t h e d i l u t e d s o l u t i o n :

#e $ant to measure the concentration of ethanoic acid in vinegar The vinegar is tooconcentrated %o at first $e must prepare a diluted solution #e prepare &:&' diluted solution

#e have a volume of vinegar of &''m( !n order to prepare &:&' diluted solution) $e must sample&'m( of vinegar *+,mot to) so tenth of the .mother solution of vinegar $hich is too concentrated/and $e add *&''-&'+ 0'm( of distilled $ater in order to prepare &''m( of &:&' diluted solution

To perform this dilution) $e must use graduated pipetteof &'m( *$ith 1 lines gauges) one bea2er*$here $e are going to sample &'m( of vinegar than2s to the graduated pipette) a pear topipette) and a gauged flas2*$ith its line mar2To begin) in a bea2er) $e pour a volume of vinegar *greater than &'m() in order to sample easily&'m( of this solution than2s to the graduated pipette of &'m(

#e reali3ed a dilution #e prepared a &:&' diluted solution


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!!" Couloured titration :

!n this first e4periment) $e are going to ma2e a coloured titration as in specialty courses#e rinse the calibrated burette $ith fe$ m( of soda *titrant solution !ts concentration is C5 +&'6&'-&mol(-& #e fill the burette $ith this solution including the et belo$ the tap

#e measure out ,8+&'' m( of the &''m( diluted

solution that $e prepared in !" $ith a graduated pipetteof &'m( $e rinse *$ith diluted solution

#e pour this volume into an 9rlenmeyer flas2 #e add 1 or drops of an appropriate indicatornamed 5T5 *5romoThymol 5lue #hen the sample is prepared) $e place a stir bar into the9rlenmeyer flas2 containing the solution #e placing the flas2 on a magnetic stirrer) $hich iss$itched on #e get the follo$ing mounting :

#e add the al2ali to the acid using the burette fairly quic2ly to get an appro4imate idea $here thesolution changes colour) this is the end point #e give the 9rlenmeyer flas2 a regular s$irl This isa rough titration #e $rite the value of poured volume of soda $hich is ,9+&1m( *+volume ofequivalence

Before the equivalence

After the equivalence


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!!!" Conductimetric titration :

#e fill bac2 the burette $ith the standard solution *titrant solution) soda #e measure out &''m( of diluted solution *as $e have done in the beginning of the first e4periment .colouredtitration/ #e put this amount in a &' bea2er and $e add &''m( of distilled $ater in thisbea2er

Then) $e prepare the conductivity probe) $e rinse it $ith distilled $ater *$ith a value at the

screen $hich approach ''' m%cm

-&

)using the right"more accurate caliber#e put the conductivity probe into the bea2er #arning) the stirrer mustn


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!," pHmetric titration :

This is the third and ultimate e4periment of this lab session #e are going to reali3e a pHmetrictitration Than2s to the instructions manual given $ith the pH-meter) $e can calibrate it *theglass electrode) before beginning measurements

%o) $e calibrate the glass electrode $ith calibrated solutions or buffers *also given $ith our

pHmeter : frstly pH 7) secondly pH =) and at least pH 7 again#e put the calibrated glass electrode into a stemmed glass that $ehad filled $ith distilled $ater

#e 2eep our electrode under voltage

Then) $e measure out &'' m( of diluted solution #e putthis amount in a bea2er) $e fill bac2 burette $ith the standardsolution) and $e put the glass electrode in the bea2er#arning the stirrer mustn


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V/ Analysis :

& ,inegar is a common household item containing acetic acid as $ell as some otherchemicals This e4periment is designed to determine the molar concentration of acetic acidin a sample of vinegar by titrating it $ith a standard solution of >a?H %o $e have thefollo$ing balanced equation :

CHC??H*aq @ >a?H*aq -A CHC??>a*aq @ H1?*l

#e can also $rite :

HC1H?1*aq @ >a?H*aq -A >aC1H?1*aq @ H1?*l

1 The equivalence point is defined as the point at $hich the number of moles of baseadded is equal to the number of moles of acid present in the solution being titrated5y accurately 2no$ing the molarity of the base and the volume of base added toreach the endpoint) the molarity of the acid can be calculated) providing the reactionhas a one to one stoichiometry of acid to base

The method for calculating the molarity of acid in the solution is based on thedefinition of the equivalence point 8t the equivalence point *endpoint:

moles of base added + moles of acid presentBurther) it is 2no$n from solution chemistry that:

molarity 6 volume + number of moles

>o$) in this titration $e 2no$ the molarity and volume of the base added and thevolume of acid titrated Therefore) to find the molarity of the acid $e use theequation:

Cbase6 ,base+ Cacid6 ,acid

%o) $e can see) according the balanced equation and these e4periments andaccording that $e said previously : that $e have"introduce chemical species instoichiometric proportions Thus) $e can say that at the equivalence point : Cai6 ,ai+ C596 ,59

!n fact) this .equation/ results from : nai+ n59because of the stoichiometric proportionsconsidering a reactionalmost complete

3) #e dra$ the conductivity curve *attached sheet and determine the equivalence point or

Concentration of ethanoic acid

introduced at the beginning

Concentration of hydroxyde ions,

introduced at the equialence !oint

Volume of

hydroxyde ions

introduced

Volume of

ethanoic acidintroduced at

the beginning

"uantity of matter of ethanoic acid

introduced at the beginning

"uantity of matter of

hydroxyde ions introduced

at the equialence !oint


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end point 9 To perform this) $e must plot the curve = f(V)of conductivity) $hich ismodeled by straight line segments Then) the equivalence point corresponds to the point of.intersection/ bet$een 1 segments of straights $e can see Here) $e get the equivalencepoint or end point 9 $hose abscissa is &=' *in m( and ordonate is '7 *in m%cm -&

---A 9*&= ; '7

%o ,Dq+&=' *appro4imately) but $e are going to e4plain the difference $ith the other

results obtained than2s to the others e4periments) in the questions &'-&& about the marginof error!t is shape in accordance $ith our values) because $e can see that in our charts of values*attached sheet $e can see a greater increase from the point 9orover) it is shape in accordance $ith data values given in t e TPbecause theconductivity of a solution is delivered by the relation *.of Eohlrausch/ :

%o) than2s to the balanced equation and data values) $e get the sum :+ (H30+) + (OH-) + (Na+) + (CH3COO-) = 349.8 + 198.6 + 50.1 + 40.9 = 0.6394 mS.cm-1 0.711 mS.cm-1 (= $hen there is ,Dq poured

= *attached sheet This time) in order to determine the equivalence point or end point 9than2s to pHmetric titration) $e are going to use the .parallel tangents method/Therefore) $e plot pH against volume of al2ali added) and in the same graph) $e $ill dra$*after)because $e use first of all the tangents method the curve dpH"d,>a?H+ f*,>a?HThe method of parallel tangents is a graphical method:

#e dra$) to either side of the pH ump) t$o tangents *T& and T1 to the curve) parallelto each other

Then) $e dra$ a line perpendicular *F to these t$o tangents: it cuts 1 points in theset$o tangents

Binally) $e plot the perpendicular bisector to the segment formed by the t$o previouspoints: it intersects the curve pH + f *,b in a point $hich is the end point8s sho$n in this schema *this is an e4ample

Therefore) $e get $ith our graph *attached sheet than2s to this method) an end point

conductiity

#onic concentration in the chemical s!ecies

(in mol$m%3 or in mol$cm%3)#onic molar conductiity

(in S$m&$mol%' or in S$cm&$mol%')


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9*,e; pHeequal to 9*&7 ; G'

) >ear end point) the pH suddenly increase brutally"strongly>ear the end point) pH is multiplied *almost by 1 !n fact) pH+' $ith a poured volumeof titrant of &1m() and increases to pH+&&' at ,+&m( *considering ,e+&

appro4imately) so bet$een &1m( and &m(

(oo2 at the area dra$n in red *attached sheet-

) Bor t e first e4periment .coloured titration/) $e found the end point *$here solutionchanges colour) $ith a volume of equivalence ,Dq+ &1 m(+,9*this is the value ofpoured volume of soda

Then) $e can use the follo$ing equation : n8+ n9Then C86 ,8+ C96 ,9

Hence : C8*&+ *C96,9" ,8

%o : C8*&+ *&'6&'-&4 &1 6&'-" *&''6&'-I+A C8*&+ ')''&1"')'&' + ')& mol(-&Bor t e second e4periment) .conudctimetric titration/)$e found the end point 9 $hose

abscissa is &=' *in m( and ordonate is '7 *in m%cm-&

so ---A 9*&= ; '7Thus) $e dDtermined that ,Dq+&=' m( *abscissa of the end point 9) in m(Then $e use a similar reasoning : C8*1+ *C96,9" ,8

I+A C8*1+ *&'6&'-& 4 &=' 6&'-" *&''6&'-

I+A C8*1+ '''&='"''&' + '&= mol(-&+ &=6&'-&mol(-&

Bor t e t ird and ultimate e4periment) .pHmetric titration/)$e found the end point 9$hose abscissa is &=' *in m(Thus) $e dDtermined that ,Dq+&=' m( *abscissa of the end point 9) in m(Then $e use a similar reasoning : C8*1+ *C96,9" ,8

I+A C8*1+ *&'6&'-&

4 &6&'-

" *&''6&'-

I+A C8*1+ '''&"''&' + '& mol(-& + &=6&'-&mol(-&

%o) $e get three different value of C8$hich are a bit similar) in a field around ')&= mol(-&and

')& mol(-e determine an average value of : *C8*&@ C8*1@ C8*" + *')&1 @ ')&=' @'&"

+ '&=1 mol(-&

I+A C8*average+ &= 6&'-& mol(-&

*) #e computed an average value of concentration of acetic acid in t e &:&' diluted solution ofvinegar) so the concentration of acetic acid in the .entire/ vinegar is equal to :C8*average6 &'' + &= 6&'-& 6&'' + &= mol(-&

+) T e acid degree of vinegar) d)is the mass *value in grams of acetic acid in &''g ofvinegar The density value of vinegar is + &' gm(-&

%o) $e 2no$ that the density is equal to the ratio bet$een the mass of the studied species

"uantity of matter of diluted

solution

"uantity of matter of

titrant solution !oured(soda !oured)

Concentration of

diluted solution

oured olume of

soda (titrant solution)

Significantfigures


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*one species and its volumeFensity*vinegar+ ass*vinegar" ,olume*vinegar

#e 2no$ that acid degree is the mass *value in grams of acetic acid in &''g of vinegar%o ass*vinegar is replaced by acid degree of vinegar) hence :

8cid-degree*vinegar+ Fensity*vinegar6 ,olume*vinegarI+A 8cid-degree*vinegar+ &' 6 &''

I+A d + &'6&'1

g *+ &''g

0

&& C?>C(J%!?> :!n regards to errors t at affected t e results of t is e4periment)$e can say that : ?ne maor factor that affected the result of this e4periment $as to strength of thesodium and sodium hydro4ide !f either of these substances is left open in the atmosphere)they begin to lose their strength Furing the e4periment) the sodium and sodiumhydro4ide $ere both left open to interact $ith the environment for some time Thus) thefinal ans$er did not match the theoretical value accurately because the strength $as$ea2ened) meaning that the numbers used to calculate the molar concentration $ere not asaccurate !n order to prevent this error from affecting the results of the e4periment) $eshould attempt to 2eep the sodium and sodium hydro4ide in an enclosed environment at alltimes) thus limiting the time it has to interact $ith the atmosphere

!n addition) the equipment used have also contributed to the error as all pieces ofapparatus have an uncertainty attached to it These uncertainties are then applied to

calculations in order to 2eep up the amount of uncertainty associated $ith the amount ofmaterial used #e have uncertainties that applied $hen the solutions $ere made *theuncertainty of mass balance that $as used to measure the amount of sodium that $asneeded to ma2e the sodium hydro4ide for the lab session that teachers prepare and thetransfer of the solution from one instrument to another These uncertainties can bereduced by using more accurate equipments) for e4ample a more accurate mass balance8lso) limiting the transfer of solution from one container to another $ill also reduce theamount of error

Human udgment also accounts for some of the error in this e4periment as the person

performing the e4periment $as required to read off many measurements from the pipetteand burette This error can be reduced by al$ays ensuring that readings are al$ays madeat eye level and that the same person ta2ing the readings is constant as udgment varies$ith each person

Binally) it depends also on the theorical values employed for the calculations : fore4ample) $e use a density value of vinegar $hich is equal to &' gm(-&) $hereas others usevalues approaching &' gm(-&

!n conclusion) this e4periment found the molar concentration of acetic acid in vinegar to beKKKKHo$ever) this value $as KKKK inaccurate due to a errors that occurred $hile conductingthe investigations

#n m- because of the

density alue of inegar

.hich is defined in g$m-%'


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10/12

Scale :

0$(mS$cm

%')


11/12

Scale :

' (m-)

0$!1 so

.ithoutunits


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