Solutions

Solutions

Question

On mixing, heptane and octane form an ideal solution. At 373 K, the vapour pressures of the two liquid components (heptane and octane) are 105 kPa and 45 kPa respectively.Vapour pressure of the solution obtained by mixing 25.0 g of heptane and 35 g of octane will be (molar mass of heptane = 100 g mol–1 and of octane =114 g mol–1)

  • 144.5 kPa

  • 72.0 kPa

  • 36.1 kPa

  •  96.2 kPa

Answer

B.

72.0 kPa

Vapour pressure of the solution can be calculated by the formula
pT = XHH + Xo.P°o. 
Therefore calculate the mole fractions of heptane and octane
pT = XHH + Xo.P°o.
straight X subscript straight H space equals fraction numerator begin display style 25 over 100 end style over denominator begin display style 25 over 100 end style plus begin display style 35 over 114 end style end fraction space equals space 0.45
straight X subscript straight O space equals space fraction numerator begin display style 35 over 114 end style over denominator begin display style 25 over 100 end style space plus begin display style 35 over 114 end style end fraction space equals space 0.55
straight p subscript straight T space equals space 0.45 space straight x space 105 space plus space 0.55 space straight x space 45 space equals space 72 space kPa

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