chemical process A quantity of methyl acetate is placed in an open, transparent,

Question

chemical process A quantity of methyl acetate is placed in an open, transparent, three-litter flask and boiled long enough to purge all air from the vapor space. The flask is then sealed and allowed to equilibrate at 30 degree C. at which temperature methyl acetate has a vapor pressure of 269 mm Hg. Visual inspection shows 10 mL of liquid methyl acetate present. What is the pressure in the flask at equilibrium? Explain your reasoning. What is the total mass (grams) of methyl acetate in the flask? What fraction is in the vapor phase ai equilibrium? The above answers would be different if the species in the vessel were ethyl acetate because methyl acetate and ethyl acetate have different vapor pressures. Give u rationale for that difference.

Explanation / Answer

a) For liquid vapour equilibrium,

pressure at a given temperature is given by clausius clapeyron equation,

ln P2/P1=Hm,vap/R[1/T1-1/T2]     where, =Hm,vap=molar heat of vasporisation

T=30+273=303K

We know Tb(b.pt of methyl acetate)=330K   , Pb(pressure at b.pt)=atmospheric pressure=760mmhg

Also using trouton’s rule, we can assume, Hm,vap/R Tb=10.5

So ln P2/P1=Hm,vap/R[1/T1-1/T2]= Hm,vap/R Tb [Tb/T-1]

Putting the values and calculating,

Log (760/p1)=10.5/2.303 [330/303-1]

Log (760/p1)=4.55(0.089)

log760/p1=0.40

760/p1=10^0.40=2.51

P1=760/2.51=302.8 mmhg

b) considering ideal behavior , Vm=RT/p =0.0821 L atm/K mol (303K)/(302.8 /760)atm

                                                                       =(24.9 L atm/mol)/0.4 atm=62.25 L/mol

Now by raoult law, P(liq)=x(liq)*P*   [partial pressure of an liq vapour is proportional to the pressure of pure component and the liquid fraction)

By dalton’s law , P gas=y (gas)* total p

At equilibrium, y (gas)* total p= x(liq)*P*  

Or y/x=p*/totalp

Or y/x=269 mmhg/302 mmhg=0.9

As total volume=3L

Volume of vapour=3L-0.01ml=2.99ml

So moles of vapour=volume/Vm=2.99L/62.5L/mol=0.048 moles vapour

Mass of vapour=0.048 mol*74.1g/mol=3.6 g

Moles of liquid=0.048 moles /0.9=0.053moles

Mass of liquid=0.053 moles*74.1 g/mol=3.92 g

c)yes, with ethyl acetate the partial pressure would be different as pressure at equilibrium is dependent on molar heat of vaporization of the liquid, which is different for different liquidsa)

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