A solution of sodium hydroxide is 8.20M and has a density of 1.20g/mL. What is the mole fraction of sodium hydroxide in the solution?
Take 1 L of solution. That will have a mass of 1000 x 1.20 g/mL = 1200 grams.
That will contain 8.20 moles NaOH or 8.20 x 40 = 328 g NaOH.
So the solution has 328 g NaOH and 1200-328 = 872 g H2O.
moles NaOH = 8.20
moles H2O = 872/molar mass H2O
XNaOH = moles NaOH/total moles.