I think so ... you're BREATHING that kind of mixture right now !
Air is a mixture of Nitrogen, Oxygen, Argon, Krypton, and a bunch of other gases in tiny amounts. The only compounds in clean air are Carbon Dioxide and water vapor.
The box stopped moving because there was a negative acceleration in the x direction caused by friction. To find the average speed, simply divide the displacement by time: 10 / 2 = 5 m/s