In mass and material balances, when would mass in not equal mass out? (give three possibilities)

Why is nh3 classified as a polar molecule

If i initially have a gas at a pressure of 12 atm, a volume of 23 liters, and a temperature of 200 k, and then i raise the pressure to 14 atm and increase the temperature to 300 k, what is the new volume of the gas?

What must occur for air masses to form fronts