CO(g) is dissolved in ${\mathrm{H}}_2\mathrm{O}$ at ${30}^{\mathrm{o}}\mathrm{C}$ and 0.020 atm. Henry's law constant for this system is $6.20\times {10}^4$ atm. Thus, mole fraction of CO(g) is

The mole fraction of a gas in solution is related to partial pressure of the gas above the solution by Henry's law. 

${\mathrm{\rho }}_{\mathrm{CO}}={\mathrm{\chi }}_{\mathrm{CO}}{\mathrm{K}}_{\mathrm{H}}$

$\therefore {\mathrm{\chi }}_{\mathrm{CO}}=\frac{{\mathrm{\rho }}_{\mathrm{CO}}}{{\mathrm{K}}_{\mathrm{H}}}=\frac{0.020\text{ atm}}{6.20\times {10}^4\text{ atm}}=3.22\times {10}^{-7}$