[xstrae]

Hi, can someone please explain how to do this problem? I dont even understand the question properly.

50 ml of a mixture of NH₃ and H₂ 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 room temperature, the mixture was shaken with alkaline pyrogallol and a contraction of 6ml  was observed. Calculate the percentage of NH₃ in the original mixture. (Assume that nitrogen does not react with Oxygen)

thanks.