Full Lab Report Experiment #2 Acid-Base Titration Lab Desc by udr50599

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									Full Lab Report Experiment #2: Acid-Base Titration           Lab Description: Acid-Base Titration      Introducti
on In this lab exercise we will evaluate the effectiveness of several indicators for the determinati
on of the point of completion of a specific acid-base neutralization reaction. We will also determin
e the unknown concentration of the strong base NaOH by its reaction with a known amount of the weak
acid, potassium acid phtalate (HKC8H4O4, abbreviated KHP). This will be accomplished using the titra
tion method. The KHP solution will be created and its volume and concentration recorded. The KHP sol
ution will be poured in a flask along with a few drops of one of three indicators we will be evaluat
ing. The NaOH solution will be poured into a buret (with volume markers) and will be used as the tit
rant. The strong base will be added slowly to the acidic solution, gradually neutralizing the acid.
The volume of base added can be determined by the difference in the initial and final volume marks o
n the buret. At a certain volume of added NaOH, all the KHP acid will be neutralized due to the larg
e equilibrium dissociation constant (Kb) of the base. This point of titration is referred to as the
equivalence point. Considering the 1:1 stoichiometry of this acid-base reaction         NaOH(aq) + C6H4(COO
H)(COOK) (aq)           C6H4(COONa)(COOK)(aq) + H2O(l)            the point of equivalence is the point of titrat
ion when the number of moles of NaOH (Na) added is equal to the number of moles of KHP (Nb) in the
solution. The number of moles of KHP in the solution can be calculated very simply by dividing the k
nown mass of the sample in the solution by its molecular mass. The unknown concentration of the NaOH
 can then be calculated in the following manner:       At the point of equivalence of a reaction of 1:1 s
toichiometric ratio, Na = Nb. The number of moles of a solute is the concentration times the volume
(N = Vc ). Thus Vaca = Vbcb. Knowing all other variables we can solve for cb by restructuring the pr
evious equation as cb = caVb/Va.         However, in order to determine the equivalence point the dissociat
ion of the indicators being used must coincide with the pH at the equivalence point. The indicator,
a weak organic acid, will dissociate at a certain pH. The dissociation of an indicator is concurrent
 with a color change or some other physical change which informs the observer of the solution's appr
oximate pH. A decreased amount of H3O+ (a product of acid dissociation) makes it more probable for t
he dissociation reaction of the indicator to occur since equilibrium must be maintained. Depending o
n the specific dissociation constant of each indicator a different H3O+ concentration (and thus pH)
will trigger the dissociation of each indicator. Since we do not know the dissociation equilibrium o
f each indicator, we cannot calculate the exact range of pH at which a color change will appear. Thu
s we will must repeat the titration experiment with a pH-meter and record the pH of the acid-base so
lution per milliliter of NaOH added. The results of this part of the experiment will be used as the
correct reference in order to determine which indicators change color at a pH range that coincides w
ith the approximate pH at the equivalence point of the given titration. The calculations of the conc
entration of NaOH must thus exclude the unsuitable indicator(s). Method and Explanations                ·Acid Bas
e Titration with Different Indicators We first created a solution of NaOH by adding 10ml of 6M NaOH
to 500 ml of distilled water. This solution was poured into a plastic bottle with a lid and was shoo
k vigorously for a few minutes. It is essential that the solution be homogeneous for the titration e
xperiment to be successful for in order to investigate and calculate the NaOH concentration it must
be constant throughout the solution. We then rinsed and dried four clean beakers and labeled them fr
om one to four. We weighed precisely 0.50 g of KHP in each beaker, with an accuracy of + .001g. The
mass of KHP added to each beaker was recorded. 50ml of distilled water was then added to each beaker
. The solution was then swirled carefully in order to dissolve the solute. Since our beakers were la
rge we were able to stir the solution contents with the magnetic stirrer without the fear of spillin
g any solution. This method was more effective and less time consuming than swirling the beaker. Thi
s solution was then poured into a 250ml Erlenmeyer's flask and a magnetic stir bar and several drops
 of phenolphthalein indicator were added. The flask was then placed on the magnetic stirrer with a w
hite paper under the flask to allow for more contrast and facilitate the detection of a color change
. Once the experimental setup was complete, a 50ml buret was rinsed twice with 10ml of the NaOH sol
ution from the plastic bottle. The buret was then filled with the NaOH solution and the initial NaOH
 volume mark was recorded. With the magnetic stirrer still on, we then placed the buret directly abo
ve the opening of the flask and slowly add NaOH to the acidic solution in the flask by slightly turn
ing the stopcock. (It is essential that the magnetic stirrer be mixing the solution continuously so
that there is no delay due to the time it take for the hydronium ions to collide with the hydroxide
ions.) The instant the color of the solution changes permanently from clear to pink the stopcock mus
t be closed and the final NaOH volume mark must be recorded. The resultant solution was then poured
into the designated waste beaker, eventually to be discarded in the waste container. The color of th
e solution should fluctuate for a few seconds from clear to pink and back again but this is simply b
ecause the equilibrium of the solution was temporarily thrown out of balance with the presence of mo
re hydroxide ions (OH-). The additional hydroxide ions neutralized the hydronium ions (H3O+) in the
solution. This causes a temporary lack of hydronium and thus shifts the dissociation equilibrium of
the indicator. The indicator dissociates temporarily revealing an instance of pink coloration. Howev
er, as there was still some KHP present, the acid, having a stronger dissociation constant, dissocia
tes and H3O+ is produced. The indicator's dissociation reaction is then forced backward and the solu
tion once again appears clear. This experiment was then repeated using two other indicators: bromoth
ymol blue and methyl orange. For the latter experiment, twice the amount of indicator drops was adde
d. This makes it easier to detect the instant the color change occurs since the methyl orange indica
tor continually and gradually changes its color. It is thus difficult to determine exactly when the
first permanent color change occurs. The gradual color changes of this indicator may be due to multi
ple steps of dissociation which may occur for example if the acid can release more than one H+ ion
(this is simply a speculation, do you know why the color change is so gradual?). The experiment was
then repeated once more. This time no indicator was used. Rather, a pH meter was used to devise a co
rrect reference with which to determine which indicators are appropriate for the determination of th
e equivalence point for this specific reaction. The experimental setup is the same as previously how
ever, we also inserted the probe of a pH meter in the flask. The probe was rinsed with distilled wat
er and dried. We then proceeded with the experiment recording the pH of the solution after every ml.
 of added NaOH. With these results we then constructed a titration curve and determined which indica
tor(s) was inappropriate for this experiment so that we may calculate the experimental concentration
 of NaOH excluding the unsuitable indicator(s).                           Results and Calculations     ·Table 1: Parts
 1 to 3 Method of TitrationTitration with Color IndicatorspH meter Sample Number#1#2#3#4 Mass of Bea
ker, (g)214.91109.86172.13186.25 Mass of Beaker and KHP sample, (g)215.41110.36172.63186.75 Mass of
the KHP sample, (g) WKHP0.50 Molecular Weight of KHP, (g/mol) MKHP204.15 Number of KHP moles in the
sample, NKHP 2.45E-3 Indicator usedPhenolphthaleinBromothymol BlueMethyl OrangeN/A Initial NaOH volu
me reading, (mL) Vin0000 Final NaOH volume reading at color change, (mL) Vfin20.521.07.023.0 NaOH vo
lume used at equivalence point, (mL) VNaOH20.521.07.023.0 Moles of KHP in sample= WKHP / MKHP = 0.50
 / 204.15 = 2.45E-3 VNaOH = Vfin - Vin            ·Observations:    IndicatorAcid colorBase color methyl orangeC
olorlessYellow* PhenolphthaleinColorlessPink bromthymol blueYellowBlue * As the solution becomes mor
e basic the methyl orange indicator becomes more orange.                                     ·Data Table 2: Part 4   Buret
Reading (mL)Ph reading 04.38 14.56 24.72 34.85 44.97 55.07 65.15 75.24 85.33 95.42 105.49 115.58 125
.65 135.74 145.83 155.92 166.03 176.15 186.32 196.57 206.97 2110.87 2211.67 2311.85                                ·Titration Cur
ve                                ·Data Table 3: Data Treatment       Sample #MKHP (mol)MNaOH at equivalence (mol)VNaOH (
L)Molarity of NaOH titrant solution calculated from results (M)Calculated [OH-] value in NaOH soluti
on used for titration (M)pH of used NaOH solution 12.45E-32.45E-30.02050.11950.119513.08 20.02100.11
670.116713.07 3*0.00700.35000.350013.54 40.02300.10650.106513.03 Averages of Appropriate Samples0.11
420.114213.06 *Indicator 3 is not suitable for the determination of the point of equivalence of this
 reaction as can be seen from the results of the pH meter experiment which we are using as our corre
ct reference.     Due to the 1:1 stoichiometric ratio of this reaction, MNaOH = MKHP at the point of eq
uivalence.     The molarity of NaOH titrant solution can be calculated using our experimental data. The
 molarity is found with the following equation: Molarity of NaOH titrant solution = MNaOH / VNaOH              S
ince NaOH is a strong acid it dissociates completely and thus [OH-] in the NaOH titrant solution is
equal to the molarity of titrant solution: [NaOH (aq)] = [OH- (aq)]        The pH of the used NaOH solutio
n can be calculated in the following manner: pH + pOH = 14 pOH = -log [OH-] pH = 14 + log [OH-] eg.
Sample 1: pH = 14 + log (0.1195) = 14 - 0.9226 = 13.0774 ~ 13.08             Sample Averages = Sum of Results o
f Appropriate Samples / # of Appropriate Samples            Discussion, Conclusions and Errors following the C
urve of Titration In this lab, four titrations were carried out. The first three titrations were car
ried out with color indicators. The fourth titration use a pH-meter and a titration curve was plotte
d for the results of the titration. The titration curve followed quite closely with the expected gra
ph shape. From the graph, we see that the initial pH of the KHP solution is 4.38. This means that KH
P is a weak acid and, titrating it with the strong base NaOH, the equivalence point should be greate
r than 7. The area in the beginning of the graph rose gradually. After the buffer region there was a
 sharp rise in pH. This steep rise coincides with the point of titration in which 20.5 mL of NaOH ti
trant had been added and the pH of the acid-base solution was about 9.08. The equivalence point was,
 as we expected for a weak acid-strong base titration, at a basic pH. Since the equivalence point of
 "chemically equivalent" reactants represents the point at which they have the same concentration in
 the solution, we know that there are 0.00245 moles of NaOH in 20.5 mL of the titrant solution. From
 this we calculated the unknown concentration of the NaOH solution. From the phenolphthalein results
, we calculated the NaOH titrant concentration to be 0.1195M. From the bromothymol blue we found the
 NaOH titrant concentration to be 0.1167M. Using methyl orange we calculated the NaOH titrant concen
tration to be 0.350. The pH-meter experiment revealed to us that the titrant NaOH concentration was
about 0.1065. We used the results of the pH-meter experiment as the correct reference. Comparing the
 results found with the three indicators to the results from the pH meter it is evident that methyl
orange is not an appropriate indicator for the given reaction. Methyl orange has a greater dissociat
ion constant and therefore dissociates (and changes color) in the presence of more hydronium ions th
an the other indicators can dissociate in. Therefore color change of methyl orange does not signal t
he point of equivalence but rather a pH of approximately 4.8. Other than the fact that methyl orange
 was an unsuitable indicator, it was difficult to determine when it first changed color permanently.
 The dissociation of methyl orange coincides with a rather gradual continuous color change. As the p
H decreases even more the indicator becomes more orange. As I have speculated earlier, the gradual c
olor changes of this indicator may be due to multiple steps of dissociation which may occur for exam
ple if the acid can release more than one H+ ion. It was also difficult to determine the exact point
 of a permanent color change with the other indicators as the new color appeared instantaneously and
 then disappeared. These fluctuating color changes are caused by the added NaOH which temporarily th
rows the solution out of equilibrium. In an attempt to regain equilibrium the indicator, a weak acid
, dissociates. However, since there is still some KHP present it dissociates when it detects the new
ly added hydroxide ions. This forces the indicator dissociation reaction back. The reason this occur
s is that KHP, while a weak acid, has a greater dissociation constant than the phenolphthalein and b
romothymol blue indicators and thus as long as it is present the indicators will not dissociate. The
se fluctuations in color change also occur with methyl orange but they are hard to recognize due to
the pale yellow color of the indicator when it first starts to dissociate. Our average calculated co
ncentration of the NaOH titrant solution was 0.1142M. The actual concentration of the NaOH titrant s
olution can actually be calculated exactly: Moles of NaOH = 6M (10ml/1000ml) Volume of solution = 50
0 + 10 ml = 510ml Molarity = Moles/ Liter = 6.0M (10ml/1000ml) / 0.510 = 0.1176M The experimental ca
lculation of the concentration was actually quite close to the actual concentration. In fact the act
ual concentration may be slightly off due to minor inaccuracies in preparing the solutions and weigh
ing. The percentage difference of the actual and average experimental results is: 100(0.1176 - 0.114
2) / 0.1176 = +2.89%. This is a very small percent error and indicates that our experiment was succe
ssful and that the chosen indicators, phenolphthalein and bromothymol blue, are suitable for this ex
periment. Assuming the inaccuracies in preparation of the solutions was minor, the most suitable ind
icator for this experiment is actually bromothymol blue with a percent error from the actual result
of only +0.77%. However, the pH-meter, which we can assume is quite accurate, had a percent error fr
om the actual results of +9.44%. Thus there must have been some error in preparing the solutions. No
netheless, Bromothymol blue is the best color indicator for this experiment as it is also the closes
t to the pH-meter results. Phenolphthalein is also a good indicator. While the percentage error of t
he NaOH titrant concentration may be about +9% from the concentration calculated with the pH-meter r
esults, the experimentally calculated pH of the solution is in all samples quite close to that found
 with the pH meter. In fact, the pH calculated from the results obtained from the Bromothymol blue e
xperiment displayed only +0.307% error from the pH found with the pH meter. There were several other
 sources of error or inaccuracies in this lab, however the errors of this lab were relatively minor.
 There is always error in weighing and measuring due to human and instrument limitations. It was har
d to get an exact reading on the pH meter because the numbers fluctuated so much. The readings, how
ever, should not have been significantly off. The pH meter had an error of +/- 0.1 units. This may h
ave caused some minor error in the titration curve. Another source of inaccuracy is in the plotting
of the titration curve. There is much estimation in the plotting of a graph without a computer. None
theless, these inaccuracies are insignificant and do not affect our results greatly. There were also
 discrepancies in determining the point of equivalence and the half-equivalence point on the curve h
owever this is merely the cause of the inevitable estimations made on the plotting of the curve. An
obvious error in determining the end point is using an incorrect indicator. This is something we wer
e aware of before the lab and we eliminated the results of the unsuitable indicator, methyl orange,
from any calculations. It is important that the chosen indicator change color somewhere in the steep
est part of the titration curve. This makes it difficult to determine the actual end point because a
 small amount of solution can change the pH quite a bit. Ideally, the solution should have been adde
d in very small increments. This would have allowed the end point determination to have been much mo
re accurate. These errors and inaccuracies are quite insignificant however and our experiment was su
ccessful in determining the NaOH titrant concentration and its pH of 13.03. We were also able to est
imate the equivalence point of titration from the curve. We found the equivalence point occurs at a
pH of approximatel 9.08. full report experiment acid base titration description acid base titration
introduction this exercise will evaluate effectiveness several indicators determination point comple
tion specific acid base neutralization reaction will also determine unknown concentration strong nao
h reaction with known amount weak potassium phtalate abbreviated this will accomplished using titrat
ion method solution created volume concentration recorded solution poured flask along with drops thr
ee indicators evaluating naoh solution poured into buret with volume markers used titrant strong add
ed slowly acidic gradually neutralizing volume added determined difference initial final marks buret
 certain added naoh neutralized large equilibrium dissociation constant this point referred equivale
nce point considering stoichiometry reaction cooh cook coona cook equivalence when number moles equa
l number moles number moles calculated very simply dividing known mass sample molecular mass unknown
 concentration then calculated following manner equivalence stoichiometric ratio solute times thus v
aca vbcb knowing other variables solve restructuring previous equation cavb however order determine
dissociation indicators being used must coincide indicator weak organic dissociate certain dissociat
ion indicator concurrent color change some other physical change which informs observer approximate
decreased amount product makes more probable indicator occur since equilibrium must maintained depen
ding specific constant each different thus trigger each since know equilibrium each cannot calculate
 exact range which color change appear thus must repeat experiment meter record milliliter results p
art experiment used correct reference order determine which color range that coincides approximate g
iven calculations exclude unsuitable method explanations different first created adding distilled wa
ter poured into plastic bottle shook vigorously minutes essential that homogeneous successful order
investigate calculate constant throughout then rinsed dried four clean beakers labeled them from fou
r weighed precisely beaker accuracy mass beaker recorded distilled water then beaker swirled careful
ly dissolve solute since beakers were large were able stir contents magnetic stirrer without fear sp
illing method more effective less time consuming than swirling into erlenmeyer flask magnetic stir s
everal drops phenolphthalein were flask placed magnetic stirrer white paper under allow more contras
t facilitate detection once experimental setup complete buret rinsed twice from plastic bottle fille
d initial mark recorded stirrer still placed directly above opening slowly acidic slightly turning s
topcock essential that mixing continuously there delay time take hydronium ions collide hydroxide io
ns instant changes permanently from clear pink stopcock closed final mark resultant designated waste
 eventually discarded waste container should fluctuate seconds clear pink back again simply because
temporarily thrown balance presence hydroxide ions additional hydroxide neutralized hydronium causes
 temporary lack hydronium shifts dissociates temporarily revealing instance pink coloration however
there still some present having stronger dissociates produced forced backward once again appears cle
ar repeated using other bromothymol blue methyl orange latter twice amount drops makes easier detect
 instant occurs methyl orange continually gradually changes difficult exactly when first permanent o
ccurs gradual changes multiple steps occur example release than simply speculation know gradual repe
ated once time rather meter devise correct reference appropriate determination specific experimental
 setup same previously however also inserted probe meter probe rinsed distilled water dried proceede
d recording after every these results constructed curve determined inappropriate calculate experimen
tal excluding unsuitable results calculations table parts titrationtitration indicatorsph sample sam
ple wkhp molecular weight mkhp nkhp usedphenolphthaleinbromothymol bluemethyl orangen initial readin
g final reading vfin vnaoh wkhp mkhp vnaoh vfin observations indicatoracid colorbase methyl orangeco
lorlessyellow phenolphthaleincolorlesspink bromthymol blueyellowblue becomes basic orange becomes da
ta table part reading curve data table data treatment mkhp mnaoh vnaoh molarity titrant calculated v
alue averages appropriate samples suitable determination seen using correct reference stoichiometric
 ratio mnaoh molarity titrant molarity found following equation mnaoh strong dissociates completely
equal following manner averages appropriate samples samples discussion conclusions errors curve four
 titrations carried first three titrations carried fourth plotted followed quite closely expected gr
aph shape graph means weak titrating should greater than area beginning graph rose gradually after b
uffer region there sharp rise steep rise coincides been about expected basic chemically equivalent r
eactants represents they have same know unknown phenolphthalein bromothymol blue found revealed abou
t comparing found three evident given greater therefore presence dissociate therefore does signal ra
ther approximately fact unsuitable difficult when changed permanently coincides rather gradual conti
nuous decreases even becomes have speculated earlier multiple steps occur example release also diffi
cult exact permanent appeared instantaneously disappeared these fluctuating caused temporarily throw
s attempt regain still some present detects newly forces back reason occurs while greater phenolphth
alein bromothymol blue long present dissociate these fluctuations they hard recognize pale yellow st
arts average actual actually exactly liter calculation actually quite close actual fact actual sligh
tly minor inaccuracies preparing solutions weighing percentage difference average very small percent
 error indicates successful chosen suitable assuming inaccuracies preparation solutions minor most s
uitable actually percent error result only assume quite accurate percent error have been preparing s
olutions nonetheless best closest good while percentage about experimentally close fact obtained dis
played only several sources inaccuracies errors relatively minor always weighing measuring human ins
trument limitations hard exact because numbers fluctuated much readings should been significantly un
its caused another source inaccuracy plotting much estimation plotting without computer nonetheless
insignificant affect greatly discrepancies determining half merely cause inevitable estimations made
 plotting obvious determining incorrect something aware before eliminated calculations important cho
sen somewhere steepest part makes because small ideally very small increments would allowed much acc
urate errors insignificant successful determining able estimate approximatelEssay, essays, termpaper
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