how to answer if this is an open system?

1568 A constantvolume tank contains a mixture of 120 g of methane (CH4) gas and 600 g of 02 at 25C and 200 kPa. The contents of the tank are now ignited, and the methane gas burns completely. If the final temperature is 1200 K, determine (a) the final pressure in the tank and (b) the heat transfer dur ing this process