Please help me with these questions, im not understanding :(

How would the number of moles (n) of o2 change if the atmospheric pressure doubled but all other variables stayed the same

When balancing a chemical equation:

3. how much energy in joules is required to evaporate .0005 kg of liquid ammonia to vapor at the same temperature? 4. how much energy ( in megajoules ) is given up by .75 kg of water at 0c when it freezes to form ice at 0c? 5. explain how heat works between and at critical temperatures?