A certain liquid has a vapor pressure of 92.0 Torr at 23.0 ∘ C and 378.0 Torr at 45.0∘C.
A. Calculate the value of Δ H ∘ vap for this liquid. Δ H ∘ vap = kJ / mol.
B. Calculate the normal boiling point of this liquid. Boiling point: ∘ C.

Respuesta :

Explanation:

(a)   The given data is as follows.

    [tex]P_{1}[/tex] = 92.0 torr,   [tex]T_{1} = 23^{o}C[/tex] = (23 + 273)K = 296 K

 [tex]P_{2}[/tex] = 378.0 torr,   [tex]T_{2} = 45^{o}C[/tex] = (45 + 273)K = 318 K

According to Clasius-Clapeyron equation,  

             [tex]ln(\frac{P_{2}}{P_{1}}) = \frac{\Delta H_{vap}}{R}[\frac{1}{T_{2}} - \frac{1}{T_{1}}[/tex]

Putting the given values into the above formula as follows.

          [tex]ln(\frac{P_{2}}{P_{1}}) = \frac{\Delta H_{vap}}{R}[\frac{1}{T_{2}} - \frac{1}{T_{1}}[/tex]

       [tex]ln(\frac{378}{92}) = \frac{\Delta H_{vap}}{8.314 J/mol K}[\frac{1}{318} - \frac{1}{296}[/tex]

      1.413 = [tex]\frac{\Delta H_{vap}}{8.314 J/mol K}[\frac{1}{318} - \frac{1}{296}[/tex]

        [tex]\Delta H_{vap}[/tex] = 2926063.008 J/mol

or,                  = 2926.063 kJ/mol

Hence, heat of vaporization for this liquid is 2926.063 kJ/mol .

(b)   It is known that normal atmospheric pressure is 760 torr. Hence, we will calculate the normal boiling point as follows.

Hence, putting this value into Clasius-Clapeyron equation as follows.

     [tex]ln (\frac{760 torr}{378 torr}) = -\frac{2926063.008 J/mol }{8.314 J/mol K} [\frac{1}{T} - \frac{1}{318}][/tex]

          0.698 = 351944.07 [\frac{(318 - T)}{318T}][/tex]

     [tex]1.98 \times 10^{-6} = \frac{(318 - T)}{318T}[/tex]

               6.306 \times 10^{-6} \times T = (318 - T)

                        T = 317.998 K

Therefore, normal boiling point of this liquid is 317.998 K.

The heat of vaporization of the liquid is 39.2 kJ and the normal boiling point is 330 K or 57 ∘C.

Using the Clausius - Clapeyron equation;

ln(P2/P1) = -ΔH∘vap/R (1/T2 - 1/T1)

P2 =  378.0 Torr

P1 = 92.0 Torr

T2 = 318 K

T1 = 296 K

R = 8.314 J/K

ΔH∘vap = ?

ln(378.0 /92.0) = -ΔH∘vap/8.314 (1/318 - 1/296)

1.413 = 0.0003ΔH∘vap/8.314

ΔH∘vap = 1.413 × 8.314/0.0003

ΔH∘vap = 39159 J or 39.2 kJ

The boiling point occurs when P2 = 760 torr (atmospheric pressure)

ln(760/378) =  39159/8.314 (1/T2 - 1/378)

2.01 = 4710  (1/T2 - 1/378)

2.01 / 4710 = (1/T2 - 1/378)

0.00043 = 1/T2 - 0.0026

1/T2 = 0.00043 +  0.0026

T2 = 330 K or 57 ∘C

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