EXPERIMENT
IE
,l
5
MolarMassof a VolatileLiquid
Themolarmassof a volatileliquid will be d€t€rmined
by measuring
whatmassof vaporof
the liquid is rcededto fill a flask of krnwn volumeu a particularternperatueandpressure.
lntroduction
Ttp most conumn instrumentfor the determinrationof molar nusses in modem chemical
researchis the masr lpcclrometer. Srch an instrunent permis very precisedetermination
of molar mass ard also gives a grcat deal of stnrctual information about the molecule
being amlyzed; this is of great help in the iCendficationof new or unknown compounds.
lvlassqpectrometers,
howwer, are exuenrly expensiveard take a geat deal of time and
efrort o calibrat€ ad maintain For this rasoq many of the classical methodsof molar
massdeterminationare still widely @lied. In this experimenl a common modification of
the ideal gas law will be used in the det€rminationof the molar mass of a liquid that is
easily waporated.
Ttte ideal gas law (PV = nRTl indicatestbat ttE observedpropertiesof a gas sample
[pressure(P),volumb(Z), ard ternperanu{Dl are dfuectlyrclatedo the quantity of gasin tlr
sample(n, rnoles).For a given containerof fixed volurne at a particular temperanueand
prcssur€,only one possibleqrnrtrity of gascan be presentin ttp cornairpr:
PV
o=m
of the massof the gassampleundersurdyin the container,the
By carefulmeasuement
molarmassof the gassamplecanb€ calculate4sincethe molar mass,M, merelyreprepermole:
sernstlrenumberof grams,g, of tbevolatilesubstance
n=fr
In.this experimenga small amount of easily volatilized liquid will be placedin a flask of
known volunrc. The flask will be heatedin a boiling water bath and will be equilibrated
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@HoughtonMifflinCo.All r(;htsreserved.
161
162 Experiment15
MolarMassof a VolatileLiquid
of the
temperanue
with atmosphericpressure.From the volumeof the flask used' the
in
gas
contained
of
pressure,
the numberof moles
boiling watir battUandthe atmospheric
vapor
with
flask
the
n nit may be calculaterl.Fromthe massof liquid r€quiredto fill
cdculated'
whenit is in theboiling waterbatL themolarmassof theliquid rnaybe
is madein thisexperimeffthatrnayafrectyour rasults.Weassume
A majorasnrmptio-n
leasrlike an
as anidrot gas.Act',ally,a vgtor behaves
that ttr vaporof Oi fiq'iO behaves
liqurdsproid; grr rfoer corditionst-at whichthevaporwould lique&. The unknown
ideal
videdln this experimenthavebeenchoseghowever,so that thevaporwill approach
gasbehador.
Safety
Precautions
at alt timeswhile in the laboratory'
Wearsafetyglasses
o Assunre
thatthevaponiof your liquid unknownarctoxic. wort in anexhaust
hoodor otherwell-ventilatedarea
a hot'
Tle unkmwnmayalsobe flanrnable.All treatingis to be performedusing
plate.
immediTheliquid unknownsmaybe hafinfiIlO skin Avoidcontactandwash
atelyif the liquid is sPilled"
r A boifingwaterbathis usedto bat rtrcliqui4 ard tberemry be a tenderp'for
inserted'
thewateiO splashwhenthc flask containingtheunknownliquid is
Exercisecaution
r Usetongsor a towelto proEctyour handsfrom tpt glassware'
a
beaker'aluminumfregjzljtfoil needleor pig oven
500-mLErlenrreyerllaskad lQQ0-mtr
(ll0oC), unknowntiquidsanple hoplate
Procedure
direcrlyin your mtebookin ink'
Recordall dataandobservations
Preparea500-rnl,Ertenrneyerflaskbycleaningtbeftask''{.fundryingitcomple
will vaporizeunderthe condiThe gask m.St be completilydy, sirrceanywaterFesent
An ovenrnaybe availablefor
thar€sults'
tionsof the eryerimcoftoa wiu'at"enetv affe.1
technique'
anotber
ilttg G n"ir. o dryress,or your instructormaydescribe
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Experiment15
Molar Mass of a VolatileLiquid
163
Cut a squarc of thick (freezer) alumirnrmfoil to serve as a cover for the flask. Trim the
edges of the foil so that it neatly covers the morth of the flask but does not extend far
down the neck.
Preparea large beaker for use as a heating bath for ttr flask The beaker must be large
enoughfor most of the flask to be coveredby boiling water wllen in the beaker.Add the
required quanity of water to the beaker. Set up the beaker on a hot plate in the exlaut
hoo4 but do mt begin to heatthe water bath yet
Weigh the dry, emptyflask with its foil coverto the nearestmg (0.001g).
Obtain an ur*nown liquid and recordits identification number.
Add 34 mL of liquid to the dry ErlenmeyerIlask. Cover tbe flask with the foil cover,
malcing sure thet the foil cover is tightly crimped arourd the rim of the flask. punch a
single small hole in thc foil cover with a needleor pin
Heat the water in the bealcr to boiting. When the water in the beakerbegiru to boil, adjrst
the temperatue of the hoplate so that the water remainsboiling but doesnot splashfrom
the beaker.
Immese the flask containing the unknown liquid in the boiling water so that most of the
flask is coverpdwith the warcr of the heatingbath (seeFigure l5-l). Clamp the neck of the
flask to maintain the flask in the boiling water.
Figure t5-1. Apparalusfor
determinationof the molarmassof a
volatileliquft1.Most of the flask
containingthe unknownlhuid must be
beneaththe surfaceof the boiling
water bath.
Watchtbe unknownliquid carefirlly.The liquid will begin to evaporaterapidly,and is
Thc amountof liquid placedin tbe flask is muchmoretbanwill be
volumcwill decreasc.
to fill theflaskwith vaporat tbe boilingwatertemperaffe.Excessvaporwill be
necessary
escapingtbmugl th pinholenade in thefoil coverof theflask.
observed
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164
Experiment15
Molar Mass of a Volatile Liquid
Wtpn it appea$ that all the unknown liquid tus vaporized, and tlp flask is filled with
vapor, continue to heat for l-2 more minutes. Then temove the flask from the boiling
water'bath; use the clamp on the neck of the flask to protectyour handsfrom the heat.
Set the flask on the lab benclr, r€move the clamp, and allow the flask to cool to rcom
temperature.Liquid will reappear in the flask as the vapor in the flask cools. While the
flask is cooling, measue and record the exact temperatue of the boiling water in the
beaker,as well as the baromefic pressurein the laboratory.
When the flask has cooled completelyto room tempenrture,carefully dry the outside of the
flask to rEmoveany droplets of water. Then weigh the flask, foil cover, and condensed
vapor to the nearestmg (0.001g).
Repeatthe determinationby adding another3-4-mL sampleof unknown liquid. Reheatthe
flask unil it is filled with vapor, cool" and reweigh the flask. The weight of the flask after
the secondsampleof unknown liquid is vaporizedshoutdagreewith the first determination
within 0.05 g. If it doesnot, do a third determination
When two acceptabledetermimtions of the weigln of vapor neededto frll the flask have
been obtained,rcmove the foil cover fiom ttp flask and clean it our
Fill the flask to the very rim with urp wabr, cover with ttrc foil cover, and weigh the flask,
cover, and water to the nearest0.1 g. Determirrcthe temperan[e of the tap wat€r in the
flask. Using the density of water at the temperanreof the water in tlp flask and the weight
of water tlp flask contrins, calculat€the exrct volurne of the flask
If no balarce with the capacity to weigh the flask when frlled with water is available, the
volume of the flask may be approximaed by pouring the water in the flask into a l-L
graduaredcylirder ard readingthe water lel,el in the cylirder.
Using the volume of the flask (in liters), the temperatureof the boiling warer barh (in
kelvins), and the barometricprcssur€(in atmospheres),calculatethe rnrmberof moles of
vapor the flask is capableof containing.R = 0.0821L atrn/mol K.
Using the weight of unknown vapor cornaired in the flash and ttp rnrmberof moles of
vapor presengcalcutatethe molar massof the unknown liquid.
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Lab Inslructor
15
Molar Mass of a Volatile Liquid
EXPERTMENT
PRE.LABORATORY QUESTIONS
l.
The methodusedin this experimentis sometimescalledthe vapor density method. Beginningwith
the ideal gasequation, show how the densityof a vapor may be determinedby this method.
2.
lf 2.31 g of the vapor of a volatile liquid is ableto fill a 49E-mLflask at 100'C and 775 mm Hg, what
is the molecular weight of the liquid? What is the densityof the vapor under theseconditions?
3.
Why is a vapor unlikely to behaveas an ideal gasnEr the temperatureat which the vapor would liquefY?
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165
Dale
Lab lnstructor
Molar Mass of a volatile Liquid
15
EXPERTMENT
RESULTS/OBSERVATIONS
Identification number of unknown liquid
Massof emptyflask and cover
SampleI
Sample2
Mass of fl askTcover/vapor
Temperatureof vapor, oC
Temperatureof vapor, K
Pressureof vapor, mm Hg
Pressureofvapor, atm
Mass of flaski/cover with water
Mass of water in flask
Temperatureof water in flask
Density of water
Volume of flask,
L
Moles of vapor in flask
Molar massof vapor
Mean value of molar massof vaPor
Density of vapor in flask
Mean value of density of vapor
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1F
168
Experiment15
Report
QUESTIONS
l.
Two methodswere describedfor determining the volume of the flask usedfor the molar m:rssdetermination. Which methodwill give a more precisedeterminationof the volume?Why?
2.
It was important that the flask be completely dry before the unknorvn liquid was addedso that water
present would not vaporize when the flask was heated. A typical single drop of liquid water has a
volume of approximately0.05 mL. Assuming the densityof liquid water is 1.0 g/ml, how many moles
of water are in one drop of liquid, and what volume would this amount of water occupy when
vaporizedat 100"Cand I atm?
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