JEE Main & Advanced JEE Main Solved Paper-2015

  • question_answer
    The vapour pressure of acetone at 200C is 185 torr. When 1.2 g of a non ? volatile substance was dissolved in 100 g of acetone at 200C, its vapour pressure was 183 torr. The molar mass (g mol?1) of the substance is [JEE Main Solved Paper-2015 ]

    A) 128                                        

    B) 488

    C) 32                                          

    D) 64

    Correct Answer: D

    Solution :

    vapour pressure of Acetone = 185 torr molar mass of Acetone\[(C{{H}_{3}}COC{{H}_{3}})=58\] \[{{n}_{acetone}}=\frac{100}{58}=1.724\] \[{{n}_{subs\tan ce}}=\frac{1.2}{M}\] by\[\frac{{{P}^{0}}-{{P}_{s}}}{{{P}^{0}}}=\frac{{{n}_{subs\tan ce}}}{{{n}_{subs\tan ce}}+{{n}_{acetone}}}\] \[\frac{185-183}{185}=\frac{\frac{1.2}{M}}{\frac{1.2}{M}+1.724}\] \[\frac{2}{185}=\frac{\frac{1.2}{M}}{\frac{1.2}{M}+1.724}\] \[\frac{185}{2}=1+\frac{1.724}{1.2/M}\] \[\frac{185-2}{2}=\frac{1.724}{1.2}M\] \[M=63.68\approx 64\]


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