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1. Solution and colligative properties — Chemistry STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceChemistry1. Solution and colligative properties1 Mark
MCQ
An ideal solution was obtained by mixing methanol and ethanol. If the partial vapour pressure of methanol and ethanol are $2.619\,kPa$ and $4.556\,kPa$ respectively, the composition of the vapour (in terms of mole fraction) will be
  • A
    $0.635 $  methanol, $ 0.365$ ethanol
  • ✓
    $0.365$  methanol, $0.635 $ ethanol
  • C
    $0.574 $ methanol, $0.326 $ ethanol
  • D
    $0.173 $ methanol, $0.827 $ ethanol
✓

Answer

Correct option: B.
$0.365$  methanol, $0.635 $ ethanol
b
Partial pressure of methanol $=2.619\, kPa$

Partial pressure of ethanol $=4.556 \,kPa$

Total pressure $=$ Partial pressure of methanol $+$ Partial pressure of ethanol

Total pressure $=2.619+4.556=7.175\, kPa$

$X _{\text {methanol: }}$ : Mole fraction of methanol

$X _{\text {ethanol: }}$ : Mole fraction of ethanol

The partial pressure of methanol $= X _{\text {methanol }} \times$ Total pressure

$\therefore X _{\text {methanol }}=\frac{2.619}{7.175}$

$=0.365$

$\therefore$ Mole fraction of ethanol $= 1 -$ Mole fraction of methanol

$=1-0.365$

$=0.635$

Hence, the correct option is $B$.

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