Ncert Exercises & Solutions

5.11 A student forgot to add the reaction mixture to the round bottomed flask at 27 °C but instead he/she placed the flask on the flame. After a lapse of time, he realized his mistake, and using a pyrometer he found the temperature of the flask was 477 °C. What fraction of air would have been expelled out?

Let the volume of the round bottomed flask be V.
Then, the volume of air inside the flask at 27° C is V.
Now,

$V_{1} = V$
$T_{1} = 27 ° C = 300 K V_{2} = ?$
$T_{2} = 477 ° C = 750 K$

According to Charles's law,

\frac{V_{1}}{T_{1}} = \frac{V_{2}}{T_{2}}

\Rightarrow V_{2} = \frac{V_{1} T_{2}}{T_{1}}

= \frac{750 V}{300}

= 2.5 V

Therefore, volume of air expelled out = 2.5 V – V = 1.5 V

Hence, fraction of air expelled out =

\frac{1.5 V}{2.5 V} = \frac{3}{5}

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