There are 2 covalent bonds between the Nitrogen and Oxygen, and 1 between the Nitrogen and Chlorine.
It should be 3.
Yes, because CO2 is carbon dioxide, and carbon dioxide contains molecules.
The states in which water occur in are ice liquid and gas
Standard temperature is 273 K
Standard pressure is 1 atm
We use the ideal gas equation to find out density of nitrogen gas in g/L
Ideal gas equation:

Molar mass of 
Pressure = 1 atm
Temperature = 273 K

= 1.25 g/L
Therefore, density of nitrogen gas at STP is 1.25 g/L
The atomic mass on the periodic table represents the sum of number of protons and number of neutrons.
Atomic mass = Number of protons + number of neutrons
Hope this helps!