A piston-cylinder device contains 2 kg of H2 and 14 kg of O2 at 150 K and

Question:

A piston-cylinder device contains 2 kg of H2 and 14 kg of O2 at 150 K and 5000 kPa. Heat is then transferred until the mixture expands at constant pressure (why does the pressure remain constant?) until the temperature rises to 200 K. Determine
The heat transfer (Q) by treating the mixture as perfect gas.