Question

I read another post on this site with this question but the answeron mastering chemistry is completely different.

A mixture of 8.0 mol of H2(g) and 4.0mol O2(g) is placed in asealed container insulated at 23degrees celcius. THe mixture isignited with a spark and reacts to form liquid water. Find thetemperature of the water. If it helps the correct answer was6500degrees celcius. I would really like to know why.