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Question

50 ml of a mixture of NH3 and H2 was completely decomposed by sparking into nitrogen and hydrogen. 40 ml of oxygen was added and the mixture was sparked again. After cooling to the room temperature, the mixture was shaken with alkaline pyrogallol solution and a contraction of 6 ml was observed. Calculate the % by volume of NH3 in the original mixture.
  1. 27%
  2. 72%
  3. 36%
  4. 54%


Solution

The correct option is B 72%
Let the volume of NH3 be x ml, then the volume of H2 would be (50x) ml.
On sparking,
NH3(g)(1/2) N2(g)+(3/2) H2(g)
Since 40 ml of O2 is added and sparked, it must have reacted with H2 to form liquid water. Since 6 ml contraction in volume is there with alkaline pyrogallol as it is known to absorb O2 , so 34 ml is the volume of O2 used up.
Total volume of H2 would be 68 ml.(2H2+O22H2O)
(50x)+32x=68,50+x2=68,x=36 ml
% volume of NH3 in the original mixture =3650×100=72%

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