The way that Cpap works is that as you breath thur your nasal mask it opens your airway to stop the blockages. With a nasal mask you are supose to keep your mouth closed because if you have your mouth open when you are asleep the air will leak out through your mouth instead of going down your air way to open it. That being said it is normal for you to be able to open your mouth when your awake and not have the air come out your mouth. This is somthing that most long term users have learned to do. Its a way of getting or relasing extra air. If you are finding that your are waking with your mouth open during the night it usually means that you need a full face mask, chin strap or that you may need a higher pressure.