Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. So, if the partial pressure of CO2 in air is 0.0003 atm, then the solubility of CO2 in water at 25°C will be:
Solubility = k * P
Solubility = 0.034 mol/L * 0.0003 atm
Solubility = 1.02e-05 mol/L
Therefore, the solubility of CO2 in water at 25°C and atmospheric pressure is 1.02e-05 mol/L.