__'
I
H
t1
:
STOICHIOMETRY
Aim
Todetermine
thevalueof n in theformulaBaClr.nHrO
usinggravimetric
analysis.
Introduction
Gravimetric
analysis
involves
accurately
measuring
themassof
a product;
volumetric
analysis
involves
accurately
measuring
volumes
of
reacting
liquids,
Thisisa simple
example
of gravimetric
analysis.
Thistechnique
involves
weighing
accurately
ratherthandoingtitrations.
Thehydrated
bariumchloride
is heateduntilall thewaterhasbeen
driven
offasin theequation:
BaClr.nHrO
+ BaCl,
+ nHrO
Fromthemasses
determined
in theexperiment,
it is possible
to calculate
therelative
number
of molesof bariumchloride
andwater;andthuscalculate
a valuefor n whichmustbea
wholenumber.
Procedure
A crucible
washeated
to driveoff anyresidual
waterand,aftercoolingin a desiccatol
it was
weighed
accurately.
Approximately
2.5g of thehydrated
bariumchloride
wasaddedto thedry
pluscontents
crucible
andthecrucible
werereweighed,
againaccurately,
Fromthis,theaccurate
massof thehydrated
bariumchloride
canbecalculated.
provides
A desiccator
a
verydryatmosphere
to
allowtheheated
barium
chloride
to coolwithout
reabsorbing
anywater.
Thehydrated
barium
chloride
washeated
in thecrucible
usinga blueBunsen
flame.The
crucible
pluscontents
wereallowed
to coolin a desiccator.The
desiccator
contains
a dryingagentwhich
removes
anymoisture
fromtheair.Thisprevents
waterbeingabsorbed
bythecrucible
and
contents
whencooling
pluscontents
in thedesiccator.
Thecooledcrucible
werethenreweighed.
Theheating,
coolingin thedesiccator,
weighing
cyclewasrepeated
untila constant
masshad
beenachieved.
At thisstage,
it waspresumed
thatallthewaterhadbeendrivenoff.
Results
= 32.67
Massof emptycrucible
g
= 35.039
Massof crucible
+ hydrated
bariumchloride
= 35'03- 32.67g = 2.369
Therefore
massof hydrated
bariumchloride
= 34'69g
Massof crucible
+ anhydrous
barium
chloride
(afterconstant
masshadbeenreached)
= 34.69- 32.67= Z.OZ7
Thereforq
massof anhydrous
bariumchloride
Massof waterdrivenoff = 2.36- 2.02= 0.34g
Number
ofmoles
ofBaClr,
n = ffi
=
ffi= 0'00970m01
Number
ofmoles
ofwater,
nffMs!= +# = 0.0189
mol
RatioofmolesofBaClr:HrO=0.00970:0.0189=1:l.g5whichisapproximately
drying
agent
Conclusion
Theformula
(n= 2),
of hydrated
barium
chloride
is BaClr.2HrO
Evaluation
Thefactthattheexperimental
resultfitstheactualformulaof hydrated
bariumchloride
suggests
thatthetechnique
is accurate
andthatthesample
of hydrated
bariumchloride
is pure.
Reasons
for an inaccurate
resultinclude:
54
.
errors
in balance
readings
.
present
impurities
in thesample
of hydrated
barium
chloride
.
notall thewaterbeingdrivenoff
I
usinga yellowflameinstead
(Ayellowflamewould
of a blueflamewhenheating
thecrucible.
resultin deposits
of sootbeingleftonthebaseof thecruciblq
affecting
themasses
measured.)
- Unit
Stoichiometry
Aim
potassium
Todetermine
thepartition
coefficient
wheniodinedistributes
itselfbetween
aqueous
iodideandcyclohexane.
lntroduction
potassium
Wheniodine
is added
to a pairof immiscible
liquids,
suchasaqueous
iodide
and
cyclohexang
it distributes
or partitions
itselfbetrrueen
thetwo liquidsandthefollowing
equilibrium
isestablished:
lr(aq)* l2(c6Hr2)
Thepartition
quantitatively
of theiodinebetween
bothliquidscanbedescribed
in termsof a partition
coefficient.
Thepartition
coefficient
isan example
of an
equilibrium
constant,
K,andtherelevant
expression
is
,,
[12(C6HrJ]
N = ---E--t----=tl2(aqlJ
Procedure
iodinein C6H12
(iodine
potassium
50cm3
of 0.050moll-1
solution
iodine
dissolved
in aqueous
iodide)
waspipetted
intoa separating
funnel.50cm3
of cyclohexane
wasthen
pipetted
intothesameseparating
funnel.
Theseparating
funnelwasstoppered
andthenshaken
vigorously
for about2 minutes.
Thecontents
wereallowed
to
settleandtwo layers
separated
out.Theupperlayer;
whichcontained
iodinein
cyclohexanq
wasa purple
colour
andthelowerlayerwhichcontained
iodine
in
potassium
theaqueous
iodidewasa reddish-brown
colour.
iodine
in
aqueous
layer
Todetermine
theconcentration
of iodinein theaqueous
solution,
thelowerlayer(theaqueous
layer)wasrunoff intoa drybeaker
and10cm3of thiswaspipetted
intoa conical
flask,Thiswas
titrated
against
0.050moll-1
sodium
thiosulphate
solution.
During
thetitration,
thebrowncolour
of theiodinesolution
became
lighter.
Whenit became'straw'
coloured,
a fewdropsof starch
indicator
solution
wereadded.
Thecolour
of thesolution
became
blue-black
andtheend-point
of thetitrationwasindicated
whenthesolution
in theconical
flaskbecame
colourless.
The
titrations
wererepeated
untiltwo concordant
results
wereobtained.
Todetermine
theconcentration
of iodinein thecyclohexang
theuppercyclohexane
layer
wasrunoffintoa drybeaker.10cm3of
thissolution
waspipetted
intoa conicalflask
and
approximately
10cm3
of deionised
waterwasadded.lhis
solution
wasthentitrated
against
0.025
moll-1sodium
thiosulphate
solution.
A fewdropsof starch
indicator
solution
wereagain
addedneartheend-point.
Since
thestarch
solution
andthecyclohexane
layerdidnotmixwell,
andshake
it wasnecessary
to stopthetitrationat intervals
theconical
flaskvigorously.Ihe
endpointof thetitrationwasonceagainindicated
whenthesolution
in theconical
flaskbecame
colourless.
untiltwo concordant
results
Thetitrationwasrepeated
wereobtained.
at the
Thecolour
change
end-point
is blue-black
to
colourless.
Results
loweraqueous
layer
= 0-050moll-t
Concentration
solution
ofsodium
thiosulphate
Rough
titration
Firsttitration
Secondtitration
(cm3)
Initialburettereading
0.10
13.20
26.20
(cm3)
Finalburettereading
13.20
26.00
39.05
Titre(cm3)
13.10
12.80
12.85
= 12.83cm3
Averaoe
of concordant
results
55