The formula for mole fraction is:
-(1)
The solubility of oxygen gas = 1.0 mmol/L (given)
1.0 mmol/L means 1.0 mmol are present in 1 L.
Converting mmol to mol:
![1.00 mmol\times \frac{1 mol}{1000 mmol} = 0.001 mol](https://tex.z-dn.net/?f=1.00%20mmol%5Ctimes%20%5Cfrac%7B1%20mol%7D%7B1000%20mmol%7D%20%3D%200.001%20mol)
So, moles of oxygen = 0.001 mol
For moles of water:
1 L of water = 1000 mL of water
Since, the density of water is 1.0 g/mL.
![Density = \frac{mass}{volume}](https://tex.z-dn.net/?f=Density%20%3D%20%5Cfrac%7Bmass%7D%7Bvolume%7D)
![Mass = 1.0 g/ml\times 1000 mL = 1000 g](https://tex.z-dn.net/?f=Mass%20%3D%201.0%20g%2Fml%5Ctimes%201000%20mL%20%3D%201000%20g)
So, the mass of water is 1000 g.
Molar mass of water = 18 g/mol.
Number of moles of water = ![\frac{1000 g}{18 g/mol} = 55.55 mol](https://tex.z-dn.net/?f=%5Cfrac%7B1000%20g%7D%7B18%20g%2Fmol%7D%20%3D%2055.55%20mol)
Substituting the values in formula (1):
![mole fraction = \frac{0.001}{55.55+0.001}](https://tex.z-dn.net/?f=mole%20fraction%20%3D%20%5Cfrac%7B0.001%7D%7B55.55%2B0.001%7D)
![mole fraction = 1.8\times 10^{-5}](https://tex.z-dn.net/?f=mole%20fraction%20%3D%201.8%5Ctimes%2010%5E%7B-5%7D)
Hence, the mole fraction is
.