[stormfrost]

Small bubbles that form on the bottom of a beaker of water being heated before boiling are due to air coming out of solution. Find the total volume of nitrogen and oxygen gas that should bubble out of 1.5 L of tap water warming from 20 °C to 70°C. Assume that the water is saturated with nitrogen and oxygen at 20°C, that all the gas bubbles come out at 70°C, and the total pressure is 1.0 atm. Use the Henry’s law constants below.
Gas      k_H (M/atm) at 20°C k_H (M/atm) at 70°C  Partial Pressure
O₂:     1.3*10^-3                   6.1*10^-4               0.21 atm
N₂:     6.1*10^-4                   3.0*10^-4               0.78 atm

What I tried:
First looking at N₂ at 20°C: 6.1*10^-4M/atm  x   0.78atm= 4.758*10^-4M
4.758*10^-4M   x    1.5L= 7.137*10^-4 mol N₂ gas
Then using PV=nRT to solve for V:
1 atm * ___L = 7.137*10^-4 mol * 0.08206 L*atm/(mol * K) * 293 K
=0.0171599L for N₂at 293K

Then doing the same for the other three, I got:
0.00987945L for N₂ at 343K
0.009845846L for O₂ at 293K
0.005408365L for O₂ at 343K
So for the total volume of all the gases I summed the four values up and got 0.0423L which is almost twice the amount of the correct answer. Also I tried switching where I used partial pressures and the 1 atm but also got the incorrect answer...does anyone see where I messed up in my work or could tip me if I'm approaching this problem in the incorrect manner?
 

[Borek]

You were given Henry's constant for 70°C for a reason. Not all gases leave the solution.

[stormfrost]

In that case, would it mean I would have to subtract the volumes I got for 70°C from the volumes at 20°C?

In that case I get 11.71793 mL (The correct answer is 20.0 mL)