Looks like we can access Groups again, I couldn't get in since this afternoon. 
1. First thing to ask is, what formula relates volume, pressure, temperature, and amount of gas? Answer: PV = nRT! The values of P and V depend on the units of the gas constant used. R = 0.08206 L atm / mol K is commonly used. From its units, we see that volume needs to be converted to liters and pressure needs to be in atmospheres, which it is. Temperature is in Kelvins. So you need to convert 1.00 X 1020 km3 to dm3, which is the volume of a liter. Once you do that, plug into the formula and solve for n, the moles of gas. Moles times Avogadro's number = number of molecules (think of the units of Avogadro's number as molecules/mol, so moles X molecules/mole = number of molecules).
1 km = 1000 m, 1 km3 = 10003 m3
1 m = 10 dm, 1 m3 = 103 dm3
Use the "factor label" or "dimensional analysis" method to make the conversion, making sure the units cancel correctly and you have dm3 remaining in the numerator after the other units cancel.
2. I think I already looked at this one �?Henry's law says that the higher the partial pressure of a gas in contact with a liquid, the greater the amount of gas there will be dissolved in the liquid. So the partial pressure of N2 is what determines which case the most N2 will be dissolved in water.
3. This is a popular problem. Here is a previous message about it:
Steve
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