Q: 50 ml. of a mixture of NH_{3} and H_{2} was completely decomposed by sparking into nitrogen and hydrogen. 40 ml of oxygen was then added and the mixture was sparked again. After cooling to the room temperature the mixture was shaken with alkaline pyro-gallol and a contraction of 6 ml. was observed. Calculate the % of NH_{3} in the original mixture.

Solution: Let the volume of NH_{3} be x ml

volume of H_{2} = (50– x) ml

On sparking,

NH_{3} → (1/2)N_{2} + (3/2)H_{2}

Since 40 ml of O_{2} is added and sparked, it must have reacted with H_{2} to form liquid water. Moreover since 6 ml contraction occurs with alkaline pyrogallol, 34 ml is the volume of O_{2} used up.

Total volume of H_{2} is 68 ( 2 H2 + O2 → 2H2O)

(50–x) + (3/2)x = 68

50 + x/2 = 68 ; x = 36

NH_{3} = 72%