Acetic acid, CH 3 COOH, forms stable pairs of molecules held together by two hyd

Question

Acetic acid, CH3COOH, forms stable pairs of molecules held together by two hydrogen bonds.

Such molecules—themselves formed by the association of two simpler molecules—are called dimers. The vapor over liquid acetic acid consists of a mixture of monomers (single acetic acid molecules) and dimers. At 100.6°C, the total pressure of vapor over liquid acetic acid is 436 mmHg. If the vapor consists of 0.630 mole fraction of the dimer, what are the masses of monomer and dimer in 1.000 L of the vapor? What is the density of the vapor?

Mass monomer =  g monomer

Mass dimer =  g dimer

Density =  g/L vapor

Explanation / Answer

According to the question,the vapor will consists of 0.630 mole of dimer of acetic acid and thus 0.370 mole of monomer of acetic acid .

molar mass of dimer = 2 x 60.05 = 120.1 g/mol

so in 0.630 mole = 75.663 g ( mass of dimer in the vapor)

molar mass of monomer = 1 x 60.05 = 60.05 g /mol

in 0.370mole = 22.2185 g (mass of monomer in the vapor)

now to calculate the density of the vapor go to joint the equations PV = RT & D = m /V

we get D = Pm / RT

    total mass m = 75.66 g + 22.22 g = 97.88 g

       P = 436 mmHg = 0.574 atm

       T = 100.6 C + 273.15 = 373.75 K

       R = 0.08206 L.atm.mol-1.K-1

               D = 0.574 x 97.88 / 0.08206 x 373.75

               = 1.832 g/L

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