Nasal Mask: Am I supposed to open my mouth?

I just got my machine and nasal mask. When I put the nasal mask on I feel air going out through my nose and out my mouth and it is a very strange experience. Am I supposed to keep my mouth closed with the nasal mask on and both inhale an exhale through my nose? If I do that won't c02 build up in the mask and kill me in my sleep?

You are supposed to keep your mouth shut...inhale and exhale through the nose.
Too much air escaping out the mouth will lower the therapy pressure and it won't be able to hold the airway open and prevent the apneas.

No worry about carbon dioxide build up. All the masks have vents where the exhaled air gets blown out into the bedroom air.
Unless someone inadvertently blocks the vent holes totally for a prolonged periods of time there is no danger of build up or killing yourself...and if you did accidentally block the vent holes and things got uncomfortable with carbon dioxide...your brain would kick in reflex mode and cause you to open you mouth and breathe and you would likely wake up. In over 9 years of this therapy...I have never had that happen.

You are supposed to keep your mouth closed. There is a vent for the CO2, so it can be safely blown away. If you close your mouth you can feel that draught with your hand.