For example: If 38.0 grams of sodium chloride is dissolved in 880. grams of water, what is the mole fraction of sodium chloride in water?

You must first convert grams to moles by dividing the compound's molar mass.
(38.0 grams sodium chloride) / (58.5 grams/mole) = 0.650 moles sodium chloride
(880. grams water) / (18.0 grams/mole) = 48.9 moles water

Now, plug it in the formula and solve.
Mole fraction of sodium chloride = 0.650 moles / (0.650 moles + 48.9 moles) = 0.0131

There is no unit used for mole fraction solutions.